sched: Limit the amount of NUMA imbalance that can exist at fork time

At fork time currently, a local node can be allowed to fill completely
and allow the periodic load balancer to fix the problem. This can be
problematic in cases where a task creates lots of threads that idle until
woken as part of a worker poll causing a memory bandwidth problem.

However, a "real" workload suffers badly from this behaviour. The workload
in question is mostly NUMA aware but spawns large numbers of threads
that act as a worker pool that can be called from anywhere. These need
to spread early to get reasonable behaviour.

This patch limits how much a local node can fill before spilling over
to another node and it will not be a universal win. Specifically,
very short-lived workloads that fit within a NUMA node would prefer
the memory bandwidth.

As I cannot describe the "real" workload, the best proxy measure I found
for illustration was a page fault microbenchmark. It's not representative
of the workload but demonstrates the hazard of the current behaviour.

pft timings
                                 5.10.0-rc2             5.10.0-rc2
                          imbalancefloat-v2          forkspread-v2
Amean     elapsed-1        46.37 (   0.00%)       46.05 *   0.69%*
Amean     elapsed-4        12.43 (   0.00%)       12.49 *  -0.47%*
Amean     elapsed-7         7.61 (   0.00%)        7.55 *   0.81%*
Amean     elapsed-12        4.79 (   0.00%)        4.80 (  -0.17%)
Amean     elapsed-21        3.13 (   0.00%)        2.89 *   7.74%*
Amean     elapsed-30        3.65 (   0.00%)        2.27 *  37.62%*
Amean     elapsed-48        3.08 (   0.00%)        2.13 *  30.69%*
Amean     elapsed-79        2.00 (   0.00%)        1.90 *   4.95%*
Amean     elapsed-80        2.00 (   0.00%)        1.90 *   4.70%*

This is showing the time to fault regions belonging to threads. The target
machine has 80 logical CPUs and two nodes. Note the ~30% gain when the
machine is approximately the point where one node becomes fully utilised.
The slower results are borderline noise.

Kernel building shows similar benefits around the same balance point.
Generally performance was either neutral or better in the tests conducted.
The main consideration with this patch is the point where fork stops
spreading a task so some workloads may benefit from different balance
points but it would be a risky tuning parameter.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20201120090630.3286-5-mgorman@techsingularity.net

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