linux-kernel - Re: [PATCH v11 3/3] sched/fair: Use candidate prev/recent_used CPU if scanning failed for cluster wakeup

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Date:   Thu, 19 Oct 2023 14:08:47 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Yicong Yang <yangyicong@...wei.com>
Cc:     peterz@...radead.org, mingo@...hat.com, juri.lelli@...hat.com,
        dietmar.eggemann@....com, tim.c.chen@...ux.intel.com,
        yu.c.chen@...el.com, gautham.shenoy@....com, mgorman@...e.de,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, rostedt@...dmis.org,
        bsegall@...gle.com, bristot@...hat.com, prime.zeng@...wei.com,
        yangyicong@...ilicon.com, jonathan.cameron@...wei.com,
        ego@...ux.vnet.ibm.com, srikar@...ux.vnet.ibm.com,
        linuxarm@...wei.com, 21cnbao@...il.com, kprateek.nayak@....com,
        wuyun.abel@...edance.com
Subject: Re: [PATCH v11 3/3] sched/fair: Use candidate prev/recent_used CPU if
 scanning failed for cluster wakeup

On Thu, 19 Oct 2023 at 05:36, Yicong Yang <yangyicong@...wei.com> wrote:
>
> From: Yicong Yang <yangyicong@...ilicon.com>
>
> Chen Yu reports a hackbench regression of cluster wakeup when
> hackbench threads equal to the CPU number [1]. Analysis shows
> it's because we wake up more on the target CPU even if the
> prev_cpu is a good wakeup candidate and leads to the decrease
> of the CPU utilization.
>
> Generally if the task's prev_cpu is idle we'll wake up the task
> on it without scanning. On cluster machines we'll try to wake up
> the task in the same cluster of the target for better cache
> affinity, so if the prev_cpu is idle but not sharing the same
> cluster with the target we'll still try to find an idle CPU within
> the cluster. This will improve the performance at low loads on
> cluster machines. But in the issue above, if the prev_cpu is idle
> but not in the cluster with the target CPU, we'll try to scan an
> idle one in the cluster. But since the system is busy, we're
> likely to fail the scanning and use target instead, even if
> the prev_cpu is idle. Then leads to the regression.
>
> This patch solves this in 2 steps:
> o record the prev_cpu/recent_used_cpu if they're good wakeup
>   candidates but not sharing the cluster with the target.
> o on scanning failure use the prev_cpu/recent_used_cpu if
>   they're recorded as idle
>
> [1] https://lore.kernel.org/all/ZGzDLuVaHR1PAYDt@chenyu5-mobl1/
>
> Reported-by: Chen Yu <yu.c.chen@...el.com>
> Closes: https://lore.kernel.org/all/ZGsLy83wPIpamy6x@chenyu5-mobl1/
> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02d842df5294..d508d1999ecc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7346,7 +7346,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>         bool has_idle_core = false;
>         struct sched_domain *sd;
>         unsigned long task_util, util_min, util_max;
> -       int i, recent_used_cpu;
> +       int i, recent_used_cpu, prev_aff = -1;
>
>         /*
>          * On asymmetric system, update task utilization because we will check
> @@ -7379,6 +7379,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>
>                 if (cpus_share_resources(prev, target))
>                         return prev;
> +
> +               prev_aff = prev;
>         }
>
>         /*
> @@ -7411,6 +7413,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>
>                 if (cpus_share_resources(recent_used_cpu, target))
>                         return recent_used_cpu;
> +       } else {
> +               recent_used_cpu = -1;
>         }
>
>         /*
> @@ -7451,6 +7455,17 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>         if ((unsigned)i < nr_cpumask_bits)
>                 return i;
>
> +       /*
> +        * For cluster machines which have lower sharing cache like L2 or
> +        * LLC Tag, we tend to find an idle CPU in the target's cluster
> +        * first. But prev_cpu or recent_used_cpu may also be a good candidate,
> +        * use them if possible when no idle CPU found in select_idle_cpu().
> +        */
> +       if ((unsigned int)prev_aff < nr_cpumask_bits)
> +               return prev_aff;
> +       if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
> +               return recent_used_cpu;
> +
>         return target;
>  }
>
> --
> 2.24.0
>