Re: [PATCH v10 3/3] sched/fair: Use candidate prev/recent_used CPU if scanning failed for cluster wakeup

[Vincent Guittot]

On Thu, 12 Oct 2023 at 14:19, Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
>
> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>
> 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 still idle
>
> [1] https://lore.kernel.org/all/ZGzDLuVaHR1PAYDt@chenyu5-mobl1/
> Reported-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4039f9b348ec..f1d94668bd71 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7392,7 +7392,7 @@ 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
> @@ -7425,6 +7425,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>
>                 if (cpus_share_resources(prev, target))
>                         return prev;
> +
> +               prev_aff = prev;
>         }
>
>         /*
> @@ -7457,6 +7459,8 @@ 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;
>         }
>
>         /*
> @@ -7497,6 +7501,19 @@ 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 &&
> +           (available_idle_cpu(prev_aff) || sched_idle_cpu(prev_aff)))

Hasn't prev_aff (i.e. prev) been already tested as idle ?

> +               return prev_aff;
> +       if ((unsigned int)recent_used_cpu < nr_cpumask_bits &&
> +           (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)))
> +               return recent_used_cpu;

same here


> +
>         return target;
>  }
>
> --
> 2.24.0
>