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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Oct 11, 2022 at 7:11 PM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> On 2022/10/12 01:00, Suren Baghdasaryan wrote:
> > On Mon, Oct 10, 2022 at 5:07 PM Chengming Zhou
> > <zhouchengming@bytedance.com> wrote:
> >>
> >> Hello,
> >>
> >> On 2022/10/11 05:21, Suren Baghdasaryan wrote:
> >>> 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.
> >>
> >> Ah, you are right, coping groupc->tasks[NR_RUNNING] and tasks[NR_IOWAIT], tasks[NR_MEMSTALL]
> >> is ok for me. (Maybe we only need to copy groupc->tasks[NR_RUNNING]?)
> >>
> >> Another way is to add an else branch:
> >>
> >>                 if (current_cpu == cpu && groupc->tasks[NR_RUNNING] == 1 &&
> >>                     !groupc->tasks[NR_IOWAIT] && !groupc->tasks[NR_MEMSTALL])
> >>                         only_avgs_work = true;
> >>                 else
> >>                         only_avgs_work = false;
> >>
> >> Both are ok for me.
> >
> > Let's use the simple way and use the tasks[] after the snapshot is taken.
>
> Ok, I changed like this:
>
>         struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
> +       int current_cpu = raw_smp_processor_id();
> +       unsigned int tasks[NR_PSI_TASK_COUNTS];
>         u64 now, state_start;
>         enum psi_states s;
>         unsigned int seq;
> @@ -256,6 +258,8 @@ 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;
> +               if (cpu == current_cpu)
> +                       memcpy(tasks, groupc->tasks, sizeof(groupc->tasks));
>         } while (read_seqcount_retry(&groupc->seq, seq));
>
> >
> >>
> >>>
> >>>>         } 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.
> >>
> >> Yes, it's safe, but has this confusion. Use a separate flag looks better, like PSI_ONLY_AVGS_WORK.
> >> But then in collect_percpu_times() we still have to clear PSI_NONIDLE of this CPU if PSI_ONLY_AVGS_WORK
> >> has been set.
> >>
> >>         for_each_possible_cpu(cpu) {
> >>                 u32 times[NR_PSI_STATES];
> >>                 u32 nonidle;
> >>                 u32 cpu_changed_states;
> >>
> >>                 get_recent_times(group, cpu, aggregator, times,
> >>                                 &cpu_changed_states);
> >>                 changed_states |= cpu_changed_states;
> >>
> >> cpu_changed_states should clear PSI_NONIDLE if PSI_ONLY_AVGS_WORK already set.
> >
> > No, PSI_NONIDLE should not be affected by PSI_ONLY_AVGS_WORK. These
> > flags should be independent and aggregated into changed_states without
> > affecting each other. Something similar to how I suggested with
> > PSI_STATE_WAKE_CLOCK in
> > https://lore.kernel.org/all/CAJuCfpFr3JfwkWbDqkU=NUJbCYuCWGySwNusMCdmS3z95WD2AQ@mail.gmail.com/#t.
>
> If we don't clear PSI_NONIDLE of this CPU, changed_states |= cpu_changed_states;
> so changed_states has PSI_NONIDLE, and we won't know if other CPUs are IDLE or not
> in psi_avgs_work().

I was thinking something like this:

--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -244,6 +244,7 @@ static void get_recent_times(struct psi_group
*group, int cpu,
        enum psi_states s;
        unsigned int seq;
        u32 state_mask;
+       bool reschedule;

        *pchanged_states = 0;

@@ -254,6 +255,14 @@ 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;
+              if (current_cpu == cpu)
+                     reschedule = groupc->tasks[NR_RUNNING] +
+                            groupc->tasks[NR_IOWAIT] +
+                            groupc->tasks[NR_MEMSTALL] > 1;
+              else
+                     reschedule = times[PSI_NONIDLE] >
+                            groupc->times_prev[aggregator][PSI_NONIDLE];
+
        } while (read_seqcount_retry(&groupc->seq, seq));

        /* Calculate state time deltas against the previous snapshot */
@@ -278,6 +287,8 @@ static void get_recent_times(struct psi_group
*group, int cpu,
               if (delta)
                      *pchanged_states |= (1 << s);
        }
+       if (reschedule)
+              *pchanged_states |= PSI_STATE_RESCHEDULE;
 }

 static void calc_avgs(unsigned long avg[3], int missed_periods,
@@ -413,7 +424,7 @@ static void psi_avgs_work(struct work_struct *work)
        struct delayed_work *dwork;
        struct psi_group *group;
        u32 changed_states;
-       bool nonidle;
+       bool reschedule;
        u64 now;

        dwork = to_delayed_work(work);
@@ -424,7 +435,7 @@ static void psi_avgs_work(struct work_struct *work)
        now = sched_clock();

        collect_percpu_times(group, PSI_AVGS, &changed_states);
-       nonidle = changed_states & (1 << PSI_NONIDLE);
+       reschedule = changed_states & (1 << PSI_STATE_RESCHEDULE);
        /*
         * If there is task activity, periodically fold the per-cpu
         * times and feed samples into the running averages. If things
@@ -435,7 +446,7 @@ static void psi_avgs_work(struct work_struct *work)
        if (now >= group->avg_next_update)
               group->avg_next_update = update_averages(group, now);

-       if (nonidle) {
+       if (reschedule) {
               schedule_delayed_work(dwork, nsecs_to_jiffies(
                             group->avg_next_update - now) + 1);
        }

Does that address your concern?

>
> Thanks.