diff options
Diffstat (limited to 'Documentation/scheduler')
| -rw-r--r-- | Documentation/scheduler/00-INDEX | 16 | ||||
| -rw-r--r-- | Documentation/scheduler/sched-arch.txt | 89 | ||||
| -rw-r--r-- | Documentation/scheduler/sched-coding.txt | 126 | ||||
| -rw-r--r-- | Documentation/scheduler/sched-design-CFS.txt | 186 | ||||
| -rw-r--r-- | Documentation/scheduler/sched-design.txt | 165 | ||||
| -rw-r--r-- | Documentation/scheduler/sched-domains.txt | 70 | ||||
| -rw-r--r-- | Documentation/scheduler/sched-nice-design.txt | 108 | ||||
| -rw-r--r-- | Documentation/scheduler/sched-stats.txt | 156 |
8 files changed, 916 insertions, 0 deletions
diff --git a/Documentation/scheduler/00-INDEX b/Documentation/scheduler/00-INDEX new file mode 100644 index 000000000000..b5f5ca069b2d --- /dev/null +++ b/Documentation/scheduler/00-INDEX @@ -0,0 +1,16 @@ +00-INDEX + - this file. +sched-arch.txt + - CPU Scheduler implementation hints for architecture specific code. +sched-coding.txt + - reference for various scheduler-related methods in the O(1) scheduler. +sched-design.txt + - goals, design and implementation of the Linux O(1) scheduler. +sched-design-CFS.txt + - goals, design and implementation of the Complete Fair Scheduler. +sched-domains.txt + - information on scheduling domains. +sched-nice-design.txt + - How and why the scheduler's nice levels are implemented. +sched-stats.txt + - information on schedstats (Linux Scheduler Statistics). diff --git a/Documentation/scheduler/sched-arch.txt b/Documentation/scheduler/sched-arch.txt new file mode 100644 index 000000000000..941615a9769b --- /dev/null +++ b/Documentation/scheduler/sched-arch.txt @@ -0,0 +1,89 @@ + CPU Scheduler implementation hints for architecture specific code + + Nick Piggin, 2005 + +Context switch +============== +1. Runqueue locking +By default, the switch_to arch function is called with the runqueue +locked. This is usually not a problem unless switch_to may need to +take the runqueue lock. This is usually due to a wake up operation in +the context switch. See include/asm-ia64/system.h for an example. + +To request the scheduler call switch_to with the runqueue unlocked, +you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file +(typically the one where switch_to is defined). + +Unlocked context switches introduce only a very minor performance +penalty to the core scheduler implementation in the CONFIG_SMP case. + +2. Interrupt status +By default, the switch_to arch function is called with interrupts +disabled. Interrupts may be enabled over the call if it is likely to +introduce a significant interrupt latency by adding the line +`#define __ARCH_WANT_INTERRUPTS_ON_CTXSW` in the same place as for +unlocked context switches. This define also implies +`__ARCH_WANT_UNLOCKED_CTXSW`. See include/asm-arm/system.h for an +example. + + +CPU idle +======== +Your cpu_idle routines need to obey the following rules: + +1. Preempt should now disabled over idle routines. Should only + be enabled to call schedule() then disabled again. + +2. need_resched/TIF_NEED_RESCHED is only ever set, and will never + be cleared until the running task has called schedule(). Idle + threads need only ever query need_resched, and may never set or + clear it. + +3. When cpu_idle finds (need_resched() == 'true'), it should call + schedule(). It should not call schedule() otherwise. + +4. The only time interrupts need to be disabled when checking + need_resched is if we are about to sleep the processor until + the next interrupt (this doesn't provide any protection of + need_resched, it prevents losing an interrupt). + + 4a. Common problem with this type of sleep appears to be: + local_irq_disable(); + if (!need_resched()) { + local_irq_enable(); + *** resched interrupt arrives here *** + __asm__("sleep until next interrupt"); + } + +5. TIF_POLLING_NRFLAG can be set by idle routines that do not + need an interrupt to wake them up when need_resched goes high. + In other words, they must be periodically polling need_resched, + although it may be reasonable to do some background work or enter + a low CPU priority. + + 5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter + an interrupt sleep, it needs to be cleared then a memory + barrier issued (followed by a test of need_resched with + interrupts disabled, as explained in 3). + +arch/i386/kernel/process.c has examples of both polling and +sleeping idle functions. + + +Possible arch/ problems +======================= + +Possible arch problems I found (and either tried to fix or didn't): + +h8300 - Is such sleeping racy vs interrupts? (See #4a). + The H8/300 manual I found indicates yes, however disabling IRQs + over the sleep mean only NMIs can wake it up, so can't fix easily + without doing spin waiting. + +ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a) + +sh64 - Is sleeping racy vs interrupts? (See #4a) + +sparc - IRQs on at this point(?), change local_irq_save to _disable. + - TODO: needs secondary CPUs to disable preempt (See #1) + diff --git a/Documentation/scheduler/sched-coding.txt b/Documentation/scheduler/sched-coding.txt new file mode 100644 index 000000000000..cbd8db752acf --- /dev/null +++ b/Documentation/scheduler/sched-coding.txt @@ -0,0 +1,126 @@ + Reference for various scheduler-related methods in the O(1) scheduler + Robert Love <rml@tech9.net>, MontaVista Software + + +Note most of these methods are local to kernel/sched.c - this is by design. +The scheduler is meant to be self-contained and abstracted away. This document +is primarily for understanding the scheduler, not interfacing to it. Some of +the discussed interfaces, however, are general process/scheduling methods. +They are typically defined in include/linux/sched.h. + + +Main Scheduling Methods +----------------------- + +void load_balance(runqueue_t *this_rq, int idle) + Attempts to pull tasks from one cpu to another to balance cpu usage, + if needed. This method is called explicitly if the runqueues are + imbalanced or periodically by the timer tick. Prior to calling, + the current runqueue must be locked and interrupts disabled. + +void schedule() + The main scheduling function. Upon return, the highest priority + process will be active. + + +Locking +------- + +Each runqueue has its own lock, rq->lock. When multiple runqueues need +to be locked, lock acquires must be ordered by ascending &runqueue value. + +A specific runqueue is locked via + + task_rq_lock(task_t pid, unsigned long *flags) + +which disables preemption, disables interrupts, and locks the runqueue pid is +running on. Likewise, + + task_rq_unlock(task_t pid, unsigned long *flags) + +unlocks the runqueue pid is running on, restores interrupts to their previous +state, and reenables preemption. + +The routines + + double_rq_lock(runqueue_t *rq1, runqueue_t *rq2) + +and + + double_rq_unlock(runqueue_t *rq1, runqueue_t *rq2) + +safely lock and unlock, respectively, the two specified runqueues. They do +not, however, disable and restore interrupts. Users are required to do so +manually before and after calls. + + +Values +------ + +MAX_PRIO + The maximum priority of the system, stored in the task as task->prio. + Lower priorities are higher. Normal (non-RT) priorities range from + MAX_RT_PRIO to (MAX_PRIO - 1). +MAX_RT_PRIO + The maximum real-time priority of the system. Valid RT priorities + range from 0 to (MAX_RT_PRIO - 1). +MAX_USER_RT_PRIO + The maximum real-time priority that is exported to user-space. Should + always be equal to or less than MAX_RT_PRIO. Setting it less allows + kernel threads to have higher priorities than any user-space task. +MIN_TIMESLICE +MAX_TIMESLICE + Respectively, the minimum and maximum timeslices (quanta) of a process. + +Data +---- + +struct runqueue + The main per-CPU runqueue data structure. +struct task_struct + The main per-process data structure. + + +General Methods +--------------- + +cpu_rq(cpu) + Returns the runqueue of the specified cpu. +this_rq() + Returns the runqueue of the current cpu. +task_rq(pid) + Returns the runqueue which holds the specified pid. +cpu_curr(cpu) + Returns the task currently running on the given cpu. +rt_task(pid) + Returns true if pid is real-time, false if not. + + +Process Control Methods +----------------------- + +void set_user_nice(task_t *p, long nice) + Sets the "nice" value of task p to the given value. +int setscheduler(pid_t pid, int policy, struct sched_param *param) + Sets the scheduling policy and parameters for the given pid. +int set_cpus_allowed(task_t *p, unsigned long new_mask) + Sets a given task's CPU affinity and migrates it to a proper cpu. + Callers must have a valid reference to the task and assure the + task not exit prematurely. No locks can be held during the call. +set_task_state(tsk, state_value) + Sets the given task's state to the given value. +set_current_state(state_value) + Sets the current task's state to the given value. +void set_tsk_need_resched(struct task_struct *tsk) + Sets need_resched in the given task. +void clear_tsk_need_resched(struct task_struct *tsk) + Clears need_resched in the given task. +void set_need_resched() + Sets need_resched in the current task. +void clear_need_resched() + Clears need_resched in the current task. +int need_resched() + Returns true if need_resched is set in the current task, false + otherwise. +yield() + Place the current process at the end of the runqueue and call schedule. diff --git a/Documentation/scheduler/sched-design-CFS.txt b/Documentation/scheduler/sched-design-CFS.txt new file mode 100644 index 000000000000..88bcb8767335 --- /dev/null +++ b/Documentation/scheduler/sched-design-CFS.txt @@ -0,0 +1,186 @@ + +This is the CFS scheduler. + +80% of CFS's design can be summed up in a single sentence: CFS basically +models an "ideal, precise multi-tasking CPU" on real hardware. + +"Ideal multi-tasking CPU" is a (non-existent :-)) CPU that has 100% +physical power and which can run each task at precise equal speed, in +parallel, each at 1/nr_running speed. For example: if there are 2 tasks +running then it runs each at 50% physical power - totally in parallel. + +On real hardware, we can run only a single task at once, so while that +one task runs, the other tasks that are waiting for the CPU are at a +disadvantage - the current task gets an unfair amount of CPU time. In +CFS this fairness imbalance is expressed and tracked via the per-task +p->wait_runtime (nanosec-unit) value. "wait_runtime" is the amount of +time the task should now run on the CPU for it to become completely fair +and balanced. + +( small detail: on 'ideal' hardware, the p->wait_runtime value would + always be zero - no task would ever get 'out of balance' from the + 'ideal' share of CPU time. ) + +CFS's task picking logic is based on this p->wait_runtime value and it +is thus very simple: it always tries to run the task with the largest +p->wait_runtime value. In other words, CFS tries to run the task with +the 'gravest need' for more CPU time. So CFS always tries to split up +CPU time between runnable tasks as close to 'ideal multitasking +hardware' as possible. + +Most of the rest of CFS's design just falls out of this really simple +concept, with a few add-on embellishments like nice levels, +multiprocessing and various algorithm variants to recognize sleepers. + +In practice it works like this: the system runs a task a bit, and when +the task schedules (or a scheduler tick happens) the task's CPU usage is +'accounted for': the (small) time it just spent using the physical CPU +is deducted from p->wait_runtime. [minus the 'fair share' it would have +gotten anyway]. Once p->wait_runtime gets low enough so that another +task becomes the 'leftmost task' of the time-ordered rbtree it maintains +(plus a small amount of 'granularity' distance relative to the leftmost +task so that we do not over-schedule tasks and trash the cache) then the +new leftmost task is picked and the current task is preempted. + +The rq->fair_clock value tracks the 'CPU time a runnable task would have +fairly gotten, had it been runnable during that time'. So by using +rq->fair_clock values we can accurately timestamp and measure the +'expected CPU time' a task should have gotten. All runnable tasks are +sorted in the rbtree by the "rq->fair_clock - p->wait_runtime" key, and +CFS picks the 'leftmost' task and sticks to it. As the system progresses +forwards, newly woken tasks are put into the tree more and more to the +right - slowly but surely giving a chance for every task to become the +'leftmost task' and thus get on the CPU within a deterministic amount of +time. + +Some implementation details: + + - the introduction of Scheduling Classes: an extensible hierarchy of + scheduler modules. These modules encapsulate scheduling policy + details and are handled by the scheduler core without the core + code assuming about them too much. + + - sched_fair.c implements the 'CFS desktop scheduler': it is a + replacement for the vanilla scheduler's SCHED_OTHER interactivity + code. + + I'd like to give credit to Con Kolivas for the general approach here: + he has proven via RSDL/SD that 'fair scheduling' is possible and that + it results in better desktop scheduling. Kudos Con! + + The CFS patch uses a completely different approach and implementation + from RSDL/SD. My goal was to make CFS's interactivity quality exceed + that of RSDL/SD, which is a high standard to meet :-) Testing + feedback is welcome to decide this one way or another. [ and, in any + case, all of SD's logic could be added via a kernel/sched_sd.c module + as well, if Con is interested in such an approach. ] + + CFS's design is quite radical: it does not use runqueues, it uses a + time-ordered rbtree to build a 'timeline' of future task execution, + and thus has no 'array switch' artifacts (by which both the vanilla + scheduler and RSDL/SD are affected). + + CFS uses nanosecond granularity accounting and does not rely on any + jiffies or other HZ detail. Thus the CFS scheduler has no notion of + 'timeslices' and has no heuristics whatsoever. There is only one + central tunable (you have to switch on CONFIG_SCHED_DEBUG): + + /proc/sys/kernel/sched_granularity_ns + + which can be used to tune the scheduler from 'desktop' (low + latencies) to 'server' (good batching) workloads. It defaults to a + setting suitable for desktop workloads. SCHED_BATCH is handled by the + CFS scheduler module too. + + Due to its design, the CFS scheduler is not prone to any of the + 'attacks' that exist today against the heuristics of the stock + scheduler: fiftyp.c, thud.c, chew.c, ring-test.c, massive_intr.c all + work fine and do not impact interactivity and produce the expected + behavior. + + the CFS scheduler has a much stronger handling of nice levels and + SCHED_BATCH: both types of workloads should be isolated much more + agressively than under the vanilla scheduler. + + ( another detail: due to nanosec accounting and timeline sorting, + sched_yield() support is very simple under CFS, and in fact under + CFS sched_yield() behaves much better than under any other + scheduler i have tested so far. ) + + - sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler + way than the vanilla scheduler does. It uses 100 runqueues (for all + 100 RT priority levels, instead of 140 in the vanilla scheduler) + and it needs no expired array. + + - reworked/sanitized SMP load-balancing: the runqueue-walking + assumptions are gone from the load-balancing code now, and + iterators of the scheduling modules are used. The balancing code got + quite a bit simpler as a result. + + +Group scheduler extension to CFS +================================ + +Normally the scheduler operates on individual tasks and strives to provide +fair CPU time to each task. Sometimes, it may be desirable to group tasks +and provide fair CPU time to each such task group. For example, it may +be desirable to first provide fair CPU time to each user on the system +and then to each task belonging to a user. + +CONFIG_FAIR_GROUP_SCHED strives to achieve exactly that. It lets +SCHED_NORMAL/BATCH tasks be be grouped and divides CPU time fairly among such +groups. At present, there are two (mutually exclusive) mechanisms to group +tasks for CPU bandwidth control purpose: + + - Based on user id (CONFIG_FAIR_USER_SCHED) + In this option, tasks are grouped according to their user id. + - Based on "cgroup" pseudo filesystem (CONFIG_FAIR_CGROUP_SCHED) + This options lets the administrator create arbitrary groups + of tasks, using the "cgroup" pseudo filesystem. See + Documentation/cgroups.txt for more information about this + filesystem. + +Only one of these options to group tasks can be chosen and not both. + +Group scheduler tunables: + +When CONFIG_FAIR_USER_SCHED is defined, a directory is created in sysfs for +each new user and a "cpu_share" file is added in that directory. + + # cd /sys/kernel/uids + # cat 512/cpu_share # Display user 512's CPU share + 1024 + # echo 2048 > 512/cpu_share # Modify user 512's CPU share + # cat 512/cpu_share # Display user 512's CPU share + 2048 + # + +CPU bandwidth between two users are divided in the ratio of their CPU shares. +For ex: if you would like user "root" to get twice the bandwidth of user +"guest", then set the cpu_share for both the users such that "root"'s +cpu_share is twice "guest"'s cpu_share + + +When CONFIG_FAIR_CGROUP_SCHED is defined, a "cpu.shares" file is created +for each group created using the pseudo filesystem. See example steps +below to create task groups and modify their CPU share using the "cgroups" +pseudo filesystem + + # mkdir /dev/cpuctl + # mount -t cgroup -ocpu none /dev/cpuctl + # cd /dev/cpuctl + + # mkdir multimedia # create "multimedia" group of tasks + # mkdir browser # create "browser" group of tasks + + # #Configure the multimedia group to receive twice the CPU bandwidth + # #that of browser group + + # echo 2048 > multimedia/cpu.shares + # echo 1024 > browser/cpu.shares + + # firefox & # Launch firefox and move it to "browser" group + # echo <firefox_pid> > browser/tasks + + # #Launch gmplayer (or your favourite movie player) + # echo <movie_player_pid> > multimedia/tasks diff --git a/Documentation/scheduler/sched-design.txt b/Documentation/scheduler/sched-design.txt new file mode 100644 index 000000000000..1605bf0cba8b --- /dev/null +++ b/Documentation/scheduler/sched-design.txt @@ -0,0 +1,165 @@ + Goals, Design and Implementation of the + new ultra-scalable O(1) scheduler + + + This is an edited version of an email Ingo Molnar sent to + lkml on 4 Jan 2002. It describes the goals, design, and + implementation of Ingo's new ultra-scalable O(1) scheduler. + Last Updated: 18 April 2002. + + +Goal +==== + +The main goal of the new scheduler is to keep all the good things we know +and love about the current Linux scheduler: + + - good interactive performance even during high load: if the user + types or clicks then the system must react instantly and must execute + the user tasks smoothly, even during considerable background load. + + - good scheduling/wakeup performance with 1-2 runnable processes. + + - fairness: no process should stay without any timeslice for any + unreasonable amount of time. No process should get an unjustly high + amount of CPU time. + + - priorities: less important tasks can be started with lower priority, + more important tasks with higher priority. + + - SMP efficiency: no CPU should stay idle if there is work to do. + + - SMP affinity: processes which run on one CPU should stay affine to + that CPU. Processes should not bounce between CPUs too frequently. + + - plus additional scheduler features: RT scheduling, CPU binding. + +and the goal is also to add a few new things: + + - fully O(1) scheduling. Are you tired of the recalculation loop + blowing the L1 cache away every now and then? Do you think the goodness + loop is taking a bit too long to finish if there are lots of runnable + processes? This new scheduler takes no prisoners: wakeup(), schedule(), + the timer interrupt are all O(1) algorithms. There is no recalculation + loop. There is no goodness loop either. + + - 'perfect' SMP scalability. With the new scheduler there is no 'big' + runqueue_lock anymore - it's all per-CPU runqueues and locks - two + tasks on two separate CPUs can wake up, schedule and context-switch + completely in parallel, without any interlocking. All + scheduling-relevant data is structured for maximum scalability. + + - better SMP affinity. The old scheduler has a particular weakness that + causes the random bouncing of tasks between CPUs if/when higher + priority/interactive tasks, this was observed and reported by many + people. The reason is that the timeslice recalculation loop first needs + every currently running task to consume its timeslice. But when this + happens on eg. an 8-way system, then this property starves an + increasing number of CPUs from executing any process. Once the last + task that has a timeslice left has finished using up that timeslice, + the recalculation loop is triggered and other CPUs can start executing + tasks again - after having idled around for a number of timer ticks. + The more CPUs, the worse this effect. + + Furthermore, this same effect causes the bouncing effect as well: + whenever there is such a 'timeslice squeeze' of the global runqueue, + idle processors start executing tasks which are not affine to that CPU. + (because the affine tasks have finished off their timeslices already.) + + The new scheduler solves this problem by distributing timeslices on a + per-CPU basis, without having any global synchronization or + recalculation. + + - batch scheduling. A significant proportion of computing-intensive tasks + benefit from batch-scheduling, where timeslices are long and processes + are roundrobin scheduled. The new scheduler does such batch-scheduling + of the lowest priority tasks - so nice +19 jobs will get + 'batch-scheduled' automatically. With this scheduler, nice +19 jobs are + in essence SCHED_IDLE, from an interactiveness point of view. + + - handle extreme loads more smoothly, without breakdown and scheduling + storms. + + - O(1) RT scheduling. For those RT folks who are paranoid about the + O(nr_running) property of the goodness loop and the recalculation loop. + + - run fork()ed children before the parent. Andrea has pointed out the + advantages of this a few months ago, but patches for this feature + do not work with the old scheduler as well as they should, + because idle processes often steal the new child before the fork()ing + CPU gets to execute it. + + +Design +====== + +The core of the new scheduler contains the following mechanisms: + + - *two* priority-ordered 'priority arrays' per CPU. There is an 'active' + array and an 'expired' array. The active array contains all tasks that + are affine to this CPU and have timeslices left. The expired array + contains all tasks which have used up their timeslices - but this array + is kept sorted as well. The active and expired array is not accessed + directly, it's accessed through two pointers in the per-CPU runqueue + structure. If all active tasks are used up then we 'switch' the two + pointers and from now on the ready-to-go (former-) expired array is the + active array - and the empty active array serves as the new collector + for expired tasks. + + - there is a 64-bit bitmap cache for array indices. Finding the highest + priority task is thus a matter of two x86 BSFL bit-search instructions. + +the split-array solution enables us to have an arbitrary number of active +and expired tasks, and the recalculation of timeslices can be done +immediately when the timeslice expires. Because the arrays are always +access through the pointers in the runqueue, switching the two arrays can +be done very quickly. + +this is a hybride priority-list approach coupled with roundrobin +scheduling and the array-switch method of distributing timeslices. + + - there is a per-task 'load estimator'. + +one of the toughest things to get right is good interactive feel during +heavy system load. While playing with various scheduler variants i found +that the best interactive feel is achieved not by 'boosting' interactive +tasks, but by 'punishing' tasks that want to use more CPU time than there +is available. This method is also much easier to do in an O(1) fashion. + +to establish the actual 'load' the task contributes to the system, a +complex-looking but pretty accurate method is used: there is a 4-entry +'history' ringbuffer of the task's activities during the last 4 seconds. +This ringbuffer is operated without much overhead. The entries tell the +scheduler a pretty accurate load-history of the task: has it used up more +CPU time or less during the past N seconds. [the size '4' and the interval +of 4x 1 seconds was found by lots of experimentation - this part is +flexible and can be changed in both directions.] + +the penalty a task gets for generating more load than the CPU can handle +is a priority decrease - there is a maximum amount to this penalty +relative to their static priority, so even fully CPU-bound tasks will +observe each other's priorities, and will share the CPU accordingly. + +the SMP load-balancer can be extended/switched with additional parallel +computing and cache hierarchy concepts: NUMA scheduling, multi-core CPUs +can be supported easily by changing the load-balancer. Right now it's +tuned for my SMP systems. + +i skipped the prev->mm == next->mm advantage - no workload i know of shows +any sensitivity to this. It can be added back by sacrificing O(1) +schedule() [the current and one-lower priority list can be searched for a +that->mm == current->mm condition], but costs a fair number of cycles +during a number of important workloads, so i wanted to avoid this as much +as possible. + +- the SMP idle-task startup code was still racy and the new scheduler +triggered this. So i streamlined the idle-setup code a bit. We do not call +into schedule() before all processors have started up fully and all idle +threads are in place. + +- the patch also cleans up a number of aspects of sched.c - moves code +into other areas of the kernel where it's appropriate, and simplifies +certain code paths and data constructs. As a result, the new scheduler's +code is smaller than the old one. + + Ingo diff --git a/Documentation/scheduler/sched-domains.txt b/Documentation/scheduler/sched-domains.txt new file mode 100644 index 000000000000..a9e990ab980f --- /dev/null +++ b/Documentation/scheduler/sched-domains.txt @@ -0,0 +1,70 @@ +Each CPU has a "base" scheduling domain (struct sched_domain). These are +accessed via cpu_sched_domain(i) and this_sched_domain() macros. The domain +hierarchy is built from these base domains via the ->parent pointer. ->parent +MUST be NULL terminated, and domain structures should be per-CPU as they +are locklessly updated. + +Each scheduling domain spans a number of CPUs (stored in the ->span field). +A domain's span MUST be a superset of it child's span (this restriction could +be relaxed if the need arises), and a base domain for CPU i MUST span at least +i. The top domain for each CPU will generally span all CPUs in the system +although strictly it doesn't have to, but this could lead to a case where some +CPUs will never be given tasks to run unless the CPUs allowed mask is +explicitly set. A sched domain's span means "balance process load among these +CPUs". + +Each scheduling domain must have one or more CPU groups (struct sched_group) +which are organised as a circular one way linked list from the ->groups +pointer. The union of cpumasks of these groups MUST be the same as the +domain's span. The intersection of cpumasks from any two of these groups +MUST be the empty set. The group pointed to by the ->groups pointer MUST +contain the CPU to which the domain belongs. Groups may be shared among +CPUs as they contain read only data after they have been set up. + +Balancing within a sched domain occurs between groups. That is, each group +is treated as one entity. The load of a group is defined as the sum of the +load of each of its member CPUs, and only when the load of a group becomes +out of balance are tasks moved between groups. + +In kernel/sched.c, rebalance_tick is run periodically on each CPU. This +function takes its CPU's base sched domain and checks to see if has reached +its rebalance interval. If so, then it will run load_balance on that domain. +rebalance_tick then checks the parent sched_domain (if it exists), and the +parent of the parent and so forth. + +*** Implementing sched domains *** +The "base" domain will "span" the first level of the hierarchy. In the case +of SMT, you'll span all siblings of the physical CPU, with each group being +a single virtual CPU. + +In SMP, the parent of the base domain will span all physical CPUs in the +node. Each group being a single physical CPU. Then with NUMA, the parent +of the SMP domain will span the entire machine, with each group having the +cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example, +might have just one domain covering its one NUMA level. + +The implementor should read comments in include/linux/sched.h: +struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of +the specifics and what to tune. + +For SMT, the architecture must define CONFIG_SCHED_SMT and provide a +cpumask_t cpu_sibling_map[NR_CPUS], where cpu_sibling_map[i] is the mask of +all "i"'s siblings as well as "i" itself. + +Architectures may retain the regular override the default SD_*_INIT flags +while using the generic domain builder in kernel/sched.c if they wish to +retain the traditional SMT->SMP->NUMA topology (or some subset of that). This +can be done by #define'ing ARCH_HASH_SCHED_TUNE. + +Alternatively, the architecture may completely override the generic domain +builder by #define'ing ARCH_HASH_SCHED_DOMAIN, and exporting your +arch_init_sched_domains function. This function will attach domains to all +CPUs using cpu_attach_domain. + +Implementors should change the line +#undef SCHED_DOMAIN_DEBUG +to +#define SCHED_DOMAIN_DEBUG +in kernel/sched.c as this enables an error checking parse of the sched domains +which should catch most possible errors (described above). It also prints out +the domain structure in a visual format. diff --git a/Documentation/scheduler/sched-nice-design.txt b/Documentation/scheduler/sched-nice-design.txt new file mode 100644 index 000000000000..e2bae5a577e3 --- /dev/null +++ b/Documentation/scheduler/sched-nice-design.txt @@ -0,0 +1,108 @@ +This document explains the thinking about the revamped and streamlined +nice-levels implementation in the new Linux scheduler. + +Nice levels were always pretty weak under Linux and people continuously +pestered us to make nice +19 tasks use up much less CPU time. + +Unfortunately that was not that easy to implement under the old +scheduler, (otherwise we'd have done it long ago) because nice level +support was historically coupled to timeslice length, and timeslice +units were driven by the HZ tick, so the smallest timeslice was 1/HZ. + +In the O(1) scheduler (in 2003) we changed negative nice levels to be +much stronger than they were before in 2.4 (and people were happy about +that change), and we also intentionally calibrated the linear timeslice +rule so that nice +19 level would be _exactly_ 1 jiffy. To better +understand it, the timeslice graph went like this (cheesy ASCII art +alert!): + + + A + \ | [timeslice length] + \ | + \ | + \ | + \ | + \|___100msecs + |^ . _ + | ^ . _ + | ^ . _ + -*----------------------------------*-----> [nice level] + -20 | +19 + | + | + +So that if someone wanted to really renice tasks, +19 would give a much +bigger hit than the normal linear rule would do. (The solution of +changing the ABI to extend priorities was discarded early on.) + +This approach worked to some degree for some time, but later on with +HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which +we felt to be a bit excessive. Excessive _not_ because it's too small of +a CPU utilization, but because it causes too frequent (once per +millisec) rescheduling. (and would thus trash the cache, etc. Remember, +this was long ago when hardware was weaker and caches were smaller, and +people were running number crunching apps at nice +19.) + +So for HZ=1000 we changed nice +19 to 5msecs, because that felt like the +right minimal granularity - and this translates to 5% CPU utilization. +But the fundamental HZ-sensitive property for nice+19 still remained, +and we never got a single complaint about nice +19 being too _weak_ in +terms of CPU utilization, we only got complaints about it (still) being +too _strong_ :-) + +To sum it up: we always wanted to make nice levels more consistent, but +within the constraints of HZ and jiffies and their nasty design level +coupling to timeslices and granularity it was not really viable. + +The second (less frequent but still periodically occuring) complaint +about Linux's nice level support was its assymetry around the origo +(which you can see demonstrated in the picture above), or more +accurately: the fact that nice level behavior depended on the _absolute_ +nice level as well, while the nice API itself is fundamentally +"relative": + + int nice(int inc); + + asmlinkage long sys_nice(int increment) + +(the first one is the glibc API, the second one is the syscall API.) +Note that the 'inc' is relative to the current nice level. Tools like +bash's "nice" command mirror this relative API. + +With the old scheduler, if you for example started a niced task with +1 +and another task with +2, the CPU split between the two tasks would +depend on the nice level of the parent shell - if it was at nice -10 the +CPU split was different than if it was at +5 or +10. + +A third complaint against Linux's nice level support was that negative +nice levels were not 'punchy enough', so lots of people had to resort to +run audio (and other multimedia) apps under RT priorities such as +SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation +proof, and a buggy SCHED_FIFO app can also lock up the system for good. + +The new scheduler in v2.6.23 addresses all three types of complaints: + +To address the first complaint (of nice levels being not "punchy" +enough), the scheduler was decoupled from 'time slice' and HZ concepts +(and granularity was made a separate concept from nice levels) and thus +it was possible to implement better and more consistent nice +19 +support: with the new scheduler nice +19 tasks get a HZ-independent +1.5%, instead of the variable 3%-5%-9% range they got in the old +scheduler. + +To address the second complaint (of nice levels not being consistent), +the new scheduler makes nice(1) have the same CPU utilization effect on +tasks, regardless of their absolute nice levels. So on the new +scheduler, running a nice +10 and a nice 11 task has the same CPU +utilization "split" between them as running a nice -5 and a nice -4 +task. (one will get 55% of the CPU, the other 45%.) That is why nice +levels were changed to be "multiplicative" (or exponential) - that way +it does not matter which nice level you start out from, the 'relative +result' will always be the same. + +The third complaint (of negative nice levels not being "punchy" enough +and forcing audio apps to run under the more dangerous SCHED_FIFO +scheduling policy) is addressed by the new scheduler almost +automatically: stronger negative nice levels are an automatic +side-effect of the recalibrated dynamic range of nice levels. diff --git a/Documentation/scheduler/sched-stats.txt b/Documentation/scheduler/sched-stats.txt new file mode 100644 index 000000000000..442e14d35dea --- /dev/null +++ b/Documentation/scheduler/sched-stats.txt @@ -0,0 +1,156 @@ +Version 14 of schedstats includes support for sched_domains, which hit the +mainline kernel in 2.6.20 although it is identical to the stats from version +12 which was in the kernel from 2.6.13-2.6.19 (version 13 never saw a kernel +release). Some counters make more sense to be per-runqueue; other to be +per-domain. Note that domains (and their associated information) will only +be pertinent and available on machines utilizing CONFIG_SMP. + +In version 14 of schedstat, there is at least one level of domain +statistics for each cpu listed, and there may well be more than one +domain. Domains have no particular names in this implementation, but +the highest numbered one typically arbitrates balancing across all the +cpus on the machine, while domain0 is the most tightly focused domain, +sometimes balancing only between pairs of cpus. At this time, there +are no architectures which need more than three domain levels. The first +field in the domain stats is a bit map indicating which cpus are affected +by that domain. + +These fields are counters, and only increment. Programs which make use +of these will need to start with a baseline observation and then calculate +the change in the counters at each subsequent observation. A perl script +which does this for many of the fields is available at + + http://eaglet.rain.com/rick/linux/schedstat/ + +Note that any such script will necessarily be version-specific, as the main +reason to change versions is changes in the output format. For those wishing +to write their own scripts, the fields are described here. + +CPU statistics +-------------- +cpu<N> 1 2 3 4 5 6 7 8 9 10 11 12 + +NOTE: In the sched_yield() statistics, the active queue is considered empty + if it has only one process in it, since obviously the process calling + sched_yield() is that process. + +First four fields are sched_yield() statistics: + 1) # of times both the active and the expired queue were empty + 2) # of times just the active queue was empty + 3) # of times just the expired queue was empty + 4) # of times sched_yield() was called + +Next three are schedule() statistics: + 5) # of times we switched to the expired queue and reused it + 6) # of times schedule() was called + 7) # of times schedule() left the processor idle + +Next two are try_to_wake_up() statistics: + 8) # of times try_to_wake_up() was called + 9) # of times try_to_wake_up() was called to wake up the local cpu + +Next three are statistics describing scheduling latency: + 10) sum of all time spent running by tasks on this processor (in jiffies) + 11) sum of all time spent waiting to run by tasks on this processor (in + jiffies) + 12) # of timeslices run on this cpu + + +Domain statistics +----------------- +One of these is produced per domain for each cpu described. (Note that if +CONFIG_SMP is not defined, *no* domains are utilized and these lines +will not appear in the output.) + +domain<N> <cpumask> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 + +The first field is a bit mask indicating what cpus this domain operates over. + +The next 24 are a variety of load_balance() statistics in grouped into types +of idleness (idle, busy, and newly idle): + + 1) # of times in this domain load_balance() was called when the + cpu was idle + 2) # of times in this domain load_balance() checked but found + the load did not require balancing when the cpu was idle + 3) # of times in this domain load_balance() tried to move one or + more tasks and failed, when the cpu was idle + 4) sum of imbalances discovered (if any) with each call to + load_balance() in this domain when the cpu was idle + 5) # of times in this domain pull_task() was called when the cpu + was idle + 6) # of times in this domain pull_task() was called even though + the target task was cache-hot when idle + 7) # of times in this domain load_balance() was called but did + not find a busier queue while the cpu was idle + 8) # of times in this domain a busier queue was found while the + cpu was idle but no busier group was found + + 9) # of times in this domain load_balance() was called when the + cpu was busy + 10) # of times in this domain load_balance() checked but found the + load did not require balancing when busy + 11) # of times in this domain load_balance() tried to move one or + more tasks and failed, when the cpu was busy + 12) sum of imbalances discovered (if any) with each call to + load_balance() in this domain when the cpu was busy + 13) # of times in this domain pull_task() was called when busy + 14) # of times in this domain pull_task() was called even though the + target task was cache-hot when busy + 15) # of times in this domain load_balance() was called but did not + find a busier queue while the cpu was busy + 16) # of times in this domain a busier queue was found while the cpu + was busy but no busier group was found + + 17) # of times in this domain load_balance() was called when the + cpu was just becoming idle + 18) # of times in this domain load_balance() checked but found the + load did not require balancing when the cpu was just becoming idle + 19) # of times in this domain load_balance() tried to move one or more + tasks and failed, when the cpu was just becoming idle + 20) sum of imbalances discovered (if any) with each call to + load_balance() in this domain when the cpu was just becoming idle + 21) # of times in this domain pull_task() was called when newly idle + 22) # of times in this domain pull_task() was called even though the + target task was cache-hot when just becoming idle + 23) # of times in this domain load_balance() was called but did not + find a busier queue while the cpu was just becoming idle + 24) # of times in this domain a busier queue was found while the cpu + was just becoming idle but no busier group was found + + Next three are active_load_balance() statistics: + 25) # of times active_load_balance() was called + 26) # of times active_load_balance() tried to move a task and failed + 27) # of times active_load_balance() successfully moved a task + + Next three are sched_balance_exec() statistics: + 28) sbe_cnt is not used + 29) sbe_balanced is not used + 30) sbe_pushed is not used + + Next three are sched_balance_fork() statistics: + 31) sbf_cnt is not used + 32) sbf_balanced is not used + 33) sbf_pushed is not used + + Next three are try_to_wake_up() statistics: + 34) # of times in this domain try_to_wake_up() awoke a task that + last ran on a different cpu in this domain + 35) # of times in this domain try_to_wake_up() moved a task to the + waking cpu because it was cache-cold on its own cpu anyway + 36) # of times in this domain try_to_wake_up() started passive balancing + +/proc/<pid>/schedstat +---------------- +schedstats also adds a new /proc/<pid/schedstat file to include some of +the same information on a per-process level. There are three fields in +this file correlating for that process to: + 1) time spent on the cpu + 2) time spent waiting on a runqueue + 3) # of timeslices run on this cpu + +A program could be easily written to make use of these extra fields to +report on how well a particular process or set of processes is faring +under the scheduler's policies. A simple version of such a program is +available at + http://eaglet.rain.com/rick/linux/schedstat/v12/latency.c |