Re: [LKP] [sched/fair] 6c8116c914: stress-ng.mmapfork.ops_per_sec -38.0% regression

[Vincent Guittot]

Hi Tao,

On Tue, 30 Jun 2020 at 11:41, Tao Zhou <ouwen210@xxxxxxxxxxx> wrote:
>
> Hi,
>
> On Tue, Jun 30, 2020 at 09:43:11AM +0200, Vincent Guittot wrote:
> > Hi Tao,
> >
> > Le lundi 15 juin 2020 Ã 16:14:01 (+0800), Xing Zhengjun a Ãcrit :
> > >
> > >
> > > On 6/15/2020 1:18 PM, Tao Zhou wrote:
> >
> > ...
> >
> > > I apply the patch based on v5.7, the regression still existed.
> >
> >
> > Could you try the patch below  ? This patch is not a real fix because it impacts performance of others benchmarks but it will at least narrow your problem.
> >
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 9f78eb76f6fb..a4d8614b1854 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8915,9 +8915,9 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> >                  * and consider staying local.
> >                  */
> >
> > -               if ((sd->flags & SD_NUMA) &&
> > -                   ((idlest_sgs.avg_load + imbalance) >= local_sgs.avg_load))
> > -                       return NULL;
> > +//             if ((sd->flags & SD_NUMA) &&
> > +//                 ((idlest_sgs.avg_load + imbalance) >= local_sgs.avg_load))
> > +//                     return NULL;
>
> Just narrow to the fork (wakeup) path that maybe lead the problem, /me think.

The perf regression seems to be fixed with this patch on my setup.
According to the statistics that I have on the use case, groups are
overloaded but load is quite low and this low level hits this NUMA
specific condition

> Some days ago, I tried this patch:
>
>   https://lore.kernel.org/lkml/20200616164801.18644-1-peter.puhov@xxxxxxxxxx/
>
> ---
>  kernel/sched/fair.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02f323b85b6d..abcbdf80ee75 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8662,8 +8662,14 @@ static bool update_pick_idlest(struct sched_group *idlest,
>
>         case group_has_spare:
>                 /* Select group with most idle CPUs */
> -               if (idlest_sgs->idle_cpus >= sgs->idle_cpus)
> +               if (idlest_sgs->idle_cpus > sgs->idle_cpus)
>                         return false;
> +
> +               /* Select group with lowest group_util */
> +               if (idlest_sgs->idle_cpus == sgs->idle_cpus &&
> +                       idlest_sgs->group_util <= sgs->group_util)
> +                       return false;
> +
>                 break;
>         }
>
> --
>
> This patch is related to wake up slow path and group type is full_busy.

I tried it but haven't seen impacts on mmapfork test results

> What I tried that got improved:
>
> $> sysbench threads --threads=16 run
>
> The total num of event(high is better):
>
> v5.8-rc1      : 34020    34494     33561
> v5.8-rc1+patch: 35466    36184     36260
>
> $> perf bench -f simple sched pipe -l 4000000
>
> v5.8-rc1      : 16.203   16.238   16.150
> v5.8-rc1+patch: 15.757   15.930   15.819
>
> I also saw some regressions about other workloads(dont know much).
> So, suggest to test this patch about this stress-ng.mmapfork. I didn't do
> this yet.
>
> Another patch i want to mention here is this(merged to V5.7 now):
>
>   commit 68f7b5cc83 ("sched/cfs: change initial value of runnable_avg")
>
> And this regression happened based on V5.7. This patch is related to fork
> wake up path of overloaded type. Absolutely need to try then.
>
> Finally, I have given a patch that seems not related to fork wake up path,
> but I also tried it on some benchmark. But, did not saw improvement there.
> I also give this changed patch here(I realized that full_busy type idle cpu
> first but not sure). Maybe not need to try.
>
> Index: core.bak/kernel/sched/fair.c
> ===================================================================
> --- core.bak.orig/kernel/sched/fair.c
> +++ core.bak/kernel/sched/fair.c
> @@ -9226,17 +9226,20 @@ static struct sched_group *find_busiest_
>                         goto out_balanced;
>
>                 if (busiest->group_weight > 1 &&
> -                   local->idle_cpus <= (busiest->idle_cpus + 1))
> -                       /*
> -                        * If the busiest group is not overloaded
> -                        * and there is no imbalance between this and busiest
> -                        * group wrt idle CPUs, it is balanced. The imbalance
> -                        * becomes significant if the diff is greater than 1
> -                        * otherwise we might end up to just move the imbalance
> -                        * on another group. Of course this applies only if
> -                        * there is more than 1 CPU per group.
> -                        */
> -                       goto out_balanced;
> +                   local->idle_cpus <= (busiest->idle_cpus + 1)) {
> +                       if (local->group_type == group_has_spare) {
> +                               /*
> +                                * If the busiest group is not overloaded
> +                                * and there is no imbalance between this and busiest
> +                                * group wrt idle CPUs, it is balanced. The imbalance
> +                                * becomes significant if the diff is greater than 1
> +                                * otherwise we might end up to just move the imbalance
> +                                * on another group. Of course this applies only if
> +                                * there is more than 1 CPU per group.
> +                                */
> +                               goto out_balanced;
> +                       }
> +               }
>
>                 if (busiest->sum_h_nr_running == 1)
>                         /*
>
>
> TBH, I don't know much about the below numbers.
>
> Thank you for the help!
>
> Thanks.
>
> >                 /*
> >                  * If the local group is less loaded than the selected
> >
> > --
> >
> >
> > > =========================================================================================
> > > tbox_group/testcase/rootfs/kconfig/compiler/nr_threads/disk/sc_pid_max/testtime/class/cpufreq_governor/ucode:
> > >
> > > lkp-bdw-ep6/stress-ng/debian-x86_64-20191114.cgz/x86_64-rhel-7.6/gcc-7/100%/1HDD/4194304/1s/scheduler/performance/0xb000038
> > >
> > > commit:
> > >   e94f80f6c49020008e6fa0f3d4b806b8595d17d8
> > >   6c8116c914b65be5e4d6f66d69c8142eb0648c22
> > >   v5.7
> > >   c7e6d37f60da32f808140b1b7dabcc3cde73c4cc  (Tao's patch)
> > >
> > > e94f80f6c4902000 6c8116c914b65be5e4d6f66d69c                        v5.7
> > > c7e6d37f60da32f808140b1b7da
> > > ---------------- --------------------------- ---------------------------
> > > ---------------------------
> > >          %stddev     %change         %stddev     %change %stddev     %change
> > > %stddev
> > >              \          |                \          |                \
> > > |                \
> > >     819250 Â  5%     -10.1%     736616 Â  8%     +41.2%    1156877 Â 3%
> > > +43.6%    1176246 Â  5%  stress-ng.futex.ops
> > >     818985 Â  5%     -10.1%     736460 Â  8%     +41.2%    1156215 Â 3%
> > > +43.6%    1176055 Â  5%  stress-ng.futex.ops_per_sec
> > >       1551 Â  3%      -3.4%       1498 Â  5%      -4.6%       1480 Â 5%
> > > -14.3%       1329 Â 11%  stress-ng.inotify.ops
> > >       1547 Â  3%      -3.5%       1492 Â  5%      -4.8%       1472 Â 5%
> > > -14.3%       1326 Â 11%  stress-ng.inotify.ops_per_sec
> > >      11292 Â  8%      -2.8%      10974 Â  8%      -9.4%      10225 Â 6%
> > > -10.1%      10146 Â  6%  stress-ng.kill.ops
> > >      11317 Â  8%      -2.6%      11023 Â  8%      -9.1%      10285 Â 5%
> > > -10.3%      10154 Â  6%  stress-ng.kill.ops_per_sec
> > >      28.20 Â  4%     -35.4%      18.22           -33.4%      18.77
> > > -27.7%      20.40 Â  9%  stress-ng.mmapfork.ops_per_sec
> > >    2999012 Â 21%     -10.1%    2696954 Â 22%     -88.5%     344447 Â 11%
> > > -87.8%     364932        stress-ng.tee.ops_per_sec
> > >       7882 Â  3%      -5.4%       7458 Â  4%      -2.0%       7724 Â 3%
> > > -2.2%       7709 Â  4%  stress-ng.vforkmany.ops
> > >       7804 Â  3%      -5.2%       7400 Â  4%      -2.0%       7647 Â 3%
> > > -2.1%       7636 Â  4%  stress-ng.vforkmany.ops_per_sec
> > >   46745421 Â  3%      -8.1%   42938569 Â  3%      -5.2%   44312072 Â 4%
> > > -2.3%   45648193        stress-ng.yield.ops
> > >   46734472 Â  3%      -8.1%   42926316 Â  3%      -5.2%   44290338 Â 4%
> > > -2.4%   45627571        stress-ng.yield.ops_per_sec
> > >
> > >
> > >
> >
> > ...
> >
> > > --
> > > Zhengjun Xing