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

[Chen Yu]

On 2023-10-13 at 17:04:32 +0200, 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 ?
>

It aims to shrink the race window that the prev idle CPU becomes non-idle during above
select_idle_cpu() scan(maybe time consuming). And it wants to make a double check.

thanks,
Chenyu
 
> > +               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
> >