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

[Vincent Guittot]

On Mon, 16 Oct 2023 at 14:55, Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
>
> Hi Vincent,
>
> On 2023/10/13 23:04, Vincent Guittot wrote:
> > 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
> >
>
> It was thought that there maybe a small potential race window here that the prev/recent_used
> CPU becoming non-idle after scanning, discussed in [1]. I think the check here won't be
> expensive so added it here. It should be redundant and can be removed.

I agree that there is a race but the whole function
select_idle_sibling() is made of possible races because by the time it
selects a CPU this one can become non-idle. It would be good to have
some figures showing that these redundant checks make a difference.

>
> [1] https://lore.kernel.org/all/ZIams6s+qShFWhfQ@xxxxxxxxxxxxxxxxxxxxxx/
>
> Thanks.
>
> >
> >> +
> >>         return target;
> >>  }
> >>
> >> --
> >> 2.24.0
> >>
> >