[PATCH 3/4] sched/fair: Do not replace recent_used_cpu with the new target

[Vincent Guittot]

On Tue, 8 Dec 2020 at 16:35, Mel Gorman <mgorman at techsingularity.net> 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 at techsingularity.net>

Reviewed-by: Vincent Guittot <vincent.guittot at linaro.org>

> ---
>  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
>