Re: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work()

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Oct 10, 2022 at 3:42 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> Pavan reported a problem that PSI avgs_work idle shutoff is not
> working at all. Because PSI_NONIDLE condition would be observed in
> psi_avgs_work()->collect_percpu_times()->get_recent_times() even if
> only the kworker running avgs_work on the CPU.
>
> Although commit 1b69ac6b40eb ("psi: fix aggregation idle shut-off")
> avoided the ping-pong wake problem when the worker sleep, psi_avgs_work()
> still will always re-arm the avgs_work, so shutoff is not working.
>
> This patch changes to consider current CPU groupc as IDLE if the
> kworker running avgs_work is the only task running and no IOWAIT
> or MEMSTALL sleep tasks, in which case we will shut off the avgs_work
> if other CPUs' groupc are also IDLE.
>
> One potential problem is that the brief period of non-idle time
> incurred between the aggregation run and the kworker's dequeue will
> be stranded in the per-cpu buckets until avgs_work run next time.
> The buckets can hold 4s worth of time, and future activity will wake
> the avgs_work with a 2s delay, giving us 2s worth of data we can leave
> behind when shut off the avgs_work. If the kworker run other works after
> avgs_work shut off and doesn't have any scheduler activities for 2s,
> this maybe a problem.
>
> Reported-by: Pavan Kondeti <quic_pkondeti@quicinc.com>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

Copying my comments from
https://lore.kernel.org/all/CAJuCfpHyYMak-mfVmtEN9Z-hGYQ6Wko57TLjukz9HaN26EDAuA@mail.gmail.com/
in case you want to continue the discussion here...

> ---
>  kernel/sched/psi.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index ee2ecc081422..f4cdf6f184ba 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -242,6 +242,8 @@ static void get_recent_times(struct psi_group *group, int cpu,
>                              u32 *pchanged_states)
>  {
>         struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
> +       int current_cpu = raw_smp_processor_id();
> +       bool only_avgs_work = false;
>         u64 now, state_start;
>         enum psi_states s;
>         unsigned int seq;
> @@ -256,6 +258,15 @@ static void get_recent_times(struct psi_group *group, int cpu,
>                 memcpy(times, groupc->times, sizeof(groupc->times));
>                 state_mask = groupc->state_mask;
>                 state_start = groupc->state_start;
> +               /*
> +                * This CPU has only avgs_work kworker running, snapshot the
> +                * newest times then don't need to re-arm for this groupc.
> +                * Normally this kworker will sleep soon and won't wake
> +                * avgs_work back up in psi_group_change().
> +                */
> +               if (current_cpu == cpu && groupc->tasks[NR_RUNNING] == 1 &&
> +                   !groupc->tasks[NR_IOWAIT] && !groupc->tasks[NR_MEMSTALL])
> +                       only_avgs_work = true;

Why do you determine only_avgs_work while taking a snapshot? The
read_seqcount_retry() might fail and the loop gets retried, which
might lead to a wrong only_avgs_work value if the state changes
between retries. I think it's safer to do this after the snapshot was
taken and to use tasks[NR_RUNNING] instead of  roupc->tasks.

>         } while (read_seqcount_retry(&groupc->seq, seq));
>
>         /* Calculate state time deltas against the previous snapshot */
> @@ -280,6 +291,10 @@ static void get_recent_times(struct psi_group *group, int cpu,
>                 if (delta)
>                         *pchanged_states |= (1 << s);
>         }
> +
> +       /* Clear PSI_NONIDLE so avgs_work won't be re-armed for this groupc */
> +       if (only_avgs_work)
> +               *pchanged_states &= ~(1 << PSI_NONIDLE);

This seems to be safe because changed_states&(1<< PSI_NONIDLE) is used
only for re-arming psi_avgs_work, however semantically this is
incorrect. The CPU was not idle when it was executing psi_avgs_work.
IMO a separate flag would avoid this confusion.

>  }
>
>  static void calc_avgs(unsigned long avg[3], int missed_periods,
> --
> 2.37.2
>