Re: [PATCH 3/4] sched/fair: Do not replace recent_used_cpu with the new target
From: Vincent Guittot
Date: Thu Dec 10 2020 - 04:41:42 EST
On Tue, 8 Dec 2020 at 17:14, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
>
> On Tue, 8 Dec 2020 at 16:35, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > After select_idle_sibling, p->recent_used_cpu is set to the
> > new target. However on the next wakeup, prev will be the same as
> > recent_used_cpu unless the load balancer has moved the task since the last
> > wakeup. It still works, but is less efficient than it can be after all
> > the changes that went in since that reduce unnecessary migrations, load
> > balancer changes etc. This patch preserves recent_used_cpu for longer.
> >
> > With tbench on a 2-socket CascadeLake machine, 80 logical CPUs, HT enabled
> >
> > 5.10.0-rc6 5.10.0-rc6
> > baseline-v2 altrecent-v2
> > Hmean 1 508.39 ( 0.00%) 502.05 * -1.25%*
> > Hmean 2 986.70 ( 0.00%) 983.65 * -0.31%*
> > Hmean 4 1914.55 ( 0.00%) 1920.24 * 0.30%*
> > Hmean 8 3702.37 ( 0.00%) 3663.96 * -1.04%*
> > Hmean 16 6573.11 ( 0.00%) 6545.58 * -0.42%*
> > Hmean 32 10142.57 ( 0.00%) 10253.73 * 1.10%*
> > Hmean 64 14348.40 ( 0.00%) 12506.31 * -12.84%*
> > Hmean 128 21842.59 ( 0.00%) 21967.13 * 0.57%*
> > Hmean 256 20813.75 ( 0.00%) 21534.52 * 3.46%*
> > Hmean 320 20684.33 ( 0.00%) 21070.14 * 1.87%*
> >
> > The different was marginal except for 64 threads which showed in the
> > baseline that the result was very unstable where as the patch was much
> > more stable. This is somewhat machine specific as on a separate 80-cpu
> > Broadwell machine the same test reported.
> >
> > 5.10.0-rc6 5.10.0-rc6
> > baseline-v2 altrecent-v2
> > Hmean 1 310.36 ( 0.00%) 291.81 * -5.98%*
> > Hmean 2 340.86 ( 0.00%) 547.22 * 60.54%*
> > Hmean 4 912.29 ( 0.00%) 1063.21 * 16.54%*
> > Hmean 8 2116.40 ( 0.00%) 2103.60 * -0.60%*
> > Hmean 16 4232.90 ( 0.00%) 4362.92 * 3.07%*
> > Hmean 32 8442.03 ( 0.00%) 8642.10 * 2.37%*
> > Hmean 64 11733.91 ( 0.00%) 11473.66 * -2.22%*
> > Hmean 128 17727.24 ( 0.00%) 16784.23 * -5.32%*
> > Hmean 256 16089.23 ( 0.00%) 16110.79 * 0.13%*
> > Hmean 320 15992.60 ( 0.00%) 16071.64 * 0.49%*
> >
> > schedstats were not used in this series but from an earlier debugging
> > effort, the schedstats after the test run were as follows;
> >
> > Ops SIS Search 5653107942.00 5726545742.00
> > Ops SIS Domain Search 3365067916.00 3319768543.00
> > Ops SIS Scanned 112173512543.00 99194352541.00
> > Ops SIS Domain Scanned 109885472517.00 96787575342.00
> > Ops SIS Failures 2923185114.00 2950166441.00
> > Ops SIS Recent Used Hit 56547.00 118064916.00
> > Ops SIS Recent Used Miss 1590899250.00 354942791.00
> > Ops SIS Recent Attempts 1590955797.00 473007707.00
> > Ops SIS Search Efficiency 5.04 5.77
> > Ops SIS Domain Search Eff 3.06 3.43
> > Ops SIS Fast Success Rate 40.47 42.03
> > Ops SIS Success Rate 48.29 48.48
> > Ops SIS Recent Success Rate 0.00 24.96
> >
> > First interesting point is the ridiculous number of times runqueues are
> > enabled -- almost 97 billion times over the course of 40 minutes
> >
> > With the patch, "Recent Used Hit" is over 2000 times more likely to
> > succeed. The failure rate also increases by quite a lot but the cost is
> > marginal even if the "Fast Success Rate" only increases by 2% overall. What
> > cannot be observed from these stats is where the biggest impact as these
> > stats cover low utilisation to over saturation.
> >
> > If graphed over time, the graphs show that the sched domain is only
> > scanned at negligible rates until the machine is fully busy. With
> > low utilisation, the "Fast Success Rate" is almost 100% until the
> > machine is fully busy. For 320 clients, the success rate is close to
> > 0% which is unsurprising.
> >
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>
> Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
This patch is responsible for a performance regression on my thx2 with
hackbench. So although i reviewed it, it should not be applied as the
change in the behavior is far deeper than expected
>
> > ---
> > kernel/sched/fair.c | 9 +--------
> > 1 file changed, 1 insertion(+), 8 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5c41875aec23..413d895bbbf8 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6277,17 +6277,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >
> > /* Check a recently used CPU as a potential idle candidate: */
> > recent_used_cpu = p->recent_used_cpu;
> > + p->recent_used_cpu = prev;
> > if (recent_used_cpu != prev &&
> > recent_used_cpu != target &&
> > cpus_share_cache(recent_used_cpu, target) &&
> > (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
> > cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) &&
> > asym_fits_capacity(task_util, recent_used_cpu)) {
> > - /*
> > - * Replace recent_used_cpu with prev as it is a potential
> > - * candidate for the next wake:
> > - */
> > - p->recent_used_cpu = prev;
> > return recent_used_cpu;
> > }
> >
> > @@ -6768,9 +6764,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> > } else if (wake_flags & WF_TTWU) { /* XXX always ? */
> > /* Fast path */
> > new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> > -
> > - if (want_affine)
> > - current->recent_used_cpu = cpu;
> > }
> > rcu_read_unlock();
> >
> > --
> > 2.26.2
> >