On Thu, Apr 25, 2019 at 5:38 AM Viresh Kumar <viresh.ku...@linaro.org> 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.ku...@linaro.org/