linux-kernel - Re: [PATCH 0/2] avoid spurious blocked load update

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Date:   Thu, 18 Nov 2021 15:40:06 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        joel@...lfernandes.org
Subject: Re: [PATCH 0/2] avoid spurious blocked load update

On Wed, 17 Nov 2021 at 00:48, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
>
> On Fri, 2021-11-12 at 10:58 +0100, Vincent Guittot wrote:
> > This patchset is a follow up of :
> > https://lore.kernel.org/lkml/20211019123537.17146-1-vincent.guittot@linaro.org/
> >
> > It ensures that newly idle load balance will not kick the update of
> > blocked load if it skips the load balance because avg_idle is too
> > short.
> > It also makes sure that rq->next_balance doesn't go in the past when
> > updated.
> >
> > Tim Chen (1):
> >   sched: sched: Fix rq->next_balance time updated to earlier than
> >     current time
> >
> > Vincent Guittot (1):
> >   sched/fair: skip newidle update stats
> >
> >  kernel/sched/fair.c | 25 ++++++++++++++++++++-----
> >  1 file changed, 20 insertions(+), 5 deletions(-)
> >
>
> Vincent,
>
> Got some data back from the benchmark team.
> To my surprise, the skip_new_idle_update_stats patch
> actually makes things a little worse.
>
>                                         Relative Performance
>                                         (higher better)
> 5.15 rc4 vanilla (cgroup disabled)      100%
> 5.15 rc4 vanilla (cgroup enabled)       96%
> patch v2                                96%
> patch v3                                96%
> patch v3
>         +skip_new_idle_update_stats     93.7%
> patch v3
>         +skip_new_idle_update_stats
>         +Fix rq->next_balance_time      93.7%
>

Yeah, that looks surprising.
patch skip_new_idle_update_stats only ensures that the cpu will not
run an update of the blocked average of idle cpus before entering idle
but outside newidle_balance if it thinks that a task is about to wake
up soon.
The end result is that we run less often
_nohz_idle_balance(cpu_rq(cpu), NOHZ_STATS_KICK, CPU_IDLE); before
entering idle.

> The cpu utilization actually is the similar compared with
> having just the v3 patch. In both cases they are
> 81% user
> 12% kernel
> 2%  idle
> 5%  waiting for IO
>
>
> Profile on key functions
> in load balancing shows a little more cpu utilization,
> which is unexpected as we are cutting short
> the newidle_balance.
>
> patch v3
>      0.56%          [k] __update_load_avg_cfs_rq
>      0.51%          [k] update_load_avg
>      0.39%          [k] update_blocked_averages
>      0.36%          [k] __update_load_avg_se
>      0.05%          [k] newidle_balance
>
> patch v3 + skip_new_idle_update_stats
>      0.58%          [k] __update_load_avg_cfs_rq
>      0.53%          [k] update_load_avg
>      0.40%          [k] update_blocked_averages
>      0.37%          [k] __update_load_avg_se
>      0.06%          [k] newidle_balance
>
> Context switch frequency is lower by 4% with the skip_new_idle_update_stats
> patch.
>
> Thanks.
>
> Tim
>