diff options
| author | Tejun Heo <tj@kernel.org> | 2014-09-23 15:24:32 -0400 |
|---|---|---|
| committer | Jens Axboe <axboe@fb.com> | 2014-09-24 08:29:36 -0600 |
| commit | 0a30288da1aec914e158c2d7a3482a85f632750f (patch) | |
| tree | 748c41c772a551dd7c94d46770aadd8a06ec8013 /lib | |
| parent | 452b6361c4d9baf6940adb7b1316e0f386c39799 (diff) | |
blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe
blk-mq uses percpu_ref for its usage counter which tracks the number
of in-flight commands and used to synchronously drain the queue on
freeze. percpu_ref shutdown takes measureable wallclock time as it
involves a sched RCU grace period. This means that draining a blk-mq
takes measureable wallclock time. One would think that this shouldn't
matter as queue shutdown should be a rare event which takes place
asynchronously w.r.t. userland.
Unfortunately, SCSI probing involves synchronously setting up and then
tearing down a lot of request_queues back-to-back for non-existent
LUNs. This means that SCSI probing may take more than ten seconds
when scsi-mq is used.
This will be properly fixed by implementing a mechanism to keep
q->mq_usage_counter in atomic mode till genhd registration; however,
that involves rather big updates to percpu_ref which is difficult to
apply late in the devel cycle (v3.17-rc6 at the moment). As a
stop-gap measure till the proper fix can be implemented in the next
cycle, this patch introduces __percpu_ref_kill_expedited() and makes
blk_mq_freeze_queue() use it. This is heavy-handed but should work
for testing the experimental SCSI blk-mq implementation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Christoph Hellwig <hch@infradead.org>
Link: http://lkml.kernel.org/g/20140919113815.GA10791@lst.de
Fixes: add703fda981 ("blk-mq: use percpu_ref for mq usage count")
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Tested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/percpu-refcount.c | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index fe5a3342e960..a89cf09a8268 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -184,3 +184,19 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref, call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu); } EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); + +/* + * XXX: Temporary kludge to work around SCSI blk-mq stall. Used only by + * block/blk-mq.c::blk_mq_freeze_queue(). Will be removed during v3.18 + * devel cycle. Do not use anywhere else. + */ +void __percpu_ref_kill_expedited(struct percpu_ref *ref) +{ + WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD, + "percpu_ref_kill() called more than once on %pf!", + ref->release); + + ref->pcpu_count_ptr |= PCPU_REF_DEAD; + synchronize_sched_expedited(); + percpu_ref_kill_rcu(&ref->rcu); +} |