Re: [RFC V2 0/2] sched/fair: Fallback to sched-idle CPU for better performance
From: Song Liu
Date: Thu May 09 2019 - 17:56:04 EST
On Thu, Apr 25, 2019 at 5:38 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> Hi,
>
> Here is another attempt to get some benefit out of the sched-idle
> policy. The previous version [1] focused on getting better power numbers
> and this version tries to get better performance or lower response time
> for the tasks.
>
> The first patch is unchanged from v1 and accumulates
> information about sched-idle tasks per CPU.
>
> The second patch changes the way the target CPU is selected in the fast
> path. Currently, we target for an idle CPU in select_idle_sibling() to
> run the next task, but in case we don't find idle CPUs it is better to
> pick a CPU which will run the task the soonest, for performance reason.
> A CPU which isn't idle but has only SCHED_IDLE activity queued on it
> should be a good target based on this criteria as any normal fair task
> will most likely preempt the currently running SCHED_IDLE task
> immediately. In fact, choosing a SCHED_IDLE CPU shall give better
> results as it should be able to run the task sooner than an idle CPU
> (which requires to be woken up from an idle state).
>
> Basic testing is done with the help of rt-app currently to make sure the
> task is getting placed correctly.
I run some test with this set on our (Facebook's) web service (main job)
and ffmpeg (side job). The result looks promising.
For all the tests below, I run the web service with same load level; and
the side job with SCHED_IDLE. I presented schedule latency distribution
of the main job. The latency distribution is measured with the runqlat tool:
https://github.com/iovisor/bpftrace/blob/master/tools/runqlat.bt
I modified the tool to only track wake up latency of the main job.
4 cases are compared here:
1. w/o this set, w/o side job;
2. w/ this set, w/o side job;
3. w/o this set, w/ side job;
4. w/ this set, w/ side job;
Case #1. w/o this set, w/o side job
@usecs:
[1] 1705 | |
[2, 4) 1102086 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[4, 8) 329160 |@@@@@@@@@@@@@@@ |
[8, 16) 34135 |@ |
[16, 32) 37466 |@ |
[32, 64) 15700 | |
[64, 128) 8759 | |
[128, 256) 5714 | |
[256, 512) 3718 | |
[512, 1K) 2152 | |
[1K, 2K) 882 | |
[2K, 4K) 256 | |
[4K, 8K) 48 | |
[8K, 16K) 2 | |
Case #2. w/ this set, w/o side job;
@usecs:
[1] 2121 | |
[2, 4) 1251877 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[4, 8) 401517 |@@@@@@@@@@@@@@@@ |
[8, 16) 64325 |@@ |
[16, 32) 74352 |@@@ |
[32, 64) 40583 |@ |
[64, 128) 26129 |@ |
[128, 256) 18612 | |
[256, 512) 12863 | |
[512, 1K) 8304 | |
[1K, 2K) 4072 | |
[2K, 4K) 1569 | |
[4K, 8K) 411 | |
[8K, 16K) 70 | |
[16K, 32K) 1 | |
>From #1 and #2, we see this set probably adds a little overhead to
scheduling when there is no side job. But the overhead is clearly very
small.
Case #3. w/o this set, w/ side job;
@usecs:
[1] 1282 | |
[2, 4) 260977 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[4, 8) 446120 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8, 16) 136927 |@@@@@@@@@@@@@@@ |
[16, 32) 148758 |@@@@@@@@@@@@@@@@@ |
[32, 64) 160291 |@@@@@@@@@@@@@@@@@@ |
[64, 128) 177292 |@@@@@@@@@@@@@@@@@@@@ |
[128, 256) 184573 |@@@@@@@@@@@@@@@@@@@@@ |
[256, 512) 173405 |@@@@@@@@@@@@@@@@@@@@ |
[512, 1K) 149662 |@@@@@@@@@@@@@@@@@ |
[1K, 2K) 120217 |@@@@@@@@@@@@@@ |
[2K, 4K) 80275 |@@@@@@@@@ |
[4K, 8K) 36108 |@@@@ |
[8K, 16K) 11121 |@ |
[16K, 32K) 736 | |
[32K, 64K) 19 | |
Case #4. w/ this set, w/ side job;
@usecs:
[1] 407 | |
[2, 4) 535378 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[4, 8) 795526 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8, 16) 93032 |@@@@@@ |
[16, 32) 89890 |@@@@@ |
[32, 64) 82775 |@@@@@ |
[64, 128) 84413 |@@@@@ |
[128, 256) 84413 |@@@@@ |
[256, 512) 77202 |@@@@@ |
[512, 1K) 66043 |@@@@ |
[1K, 2K) 49276 |@@@ |
[2K, 4K) 30114 |@ |
[4K, 8K) 11145 | |
[8K, 16K) 2328 | |
[16K, 32K) 88 | |
#3 and #4 clearly showed the benefit of this set. With this set, we see
significantly fewer latency values in the 8usecs+ ranges.
Thanks,
Song
>
> --
> viresh
>
> Viresh Kumar (2):
> sched: Start tracking SCHED_IDLE tasks count in cfs_rq
> sched/fair: Fallback to sched-idle CPU if idle CPU isn't found
>
> kernel/sched/fair.c | 42 +++++++++++++++++++++++++++++++++---------
> kernel/sched/sched.h | 2 ++
> 2 files changed, 35 insertions(+), 9 deletions(-)
>
> --
> 2.21.0.rc0.269.g1a574e7a288b
>
> [1] https://lore.kernel.org/lkml/cover.1543229820.git.viresh.kumar@xxxxxxxxxx/