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Date:   Sun, 9 Oct 2022 21:17:34 +0800
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Subject: Re: PSI idle-shutoff
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From:   Chengming Zhou <zhouchengming@bytedance.com>
To:     Suren Baghdasaryan <surenb@google.com>,
        Pavan Kondeti <quic_pkondeti@quicinc.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Charan Teja Kalla <quic_charante@quicinc.com>
References: <20220913140817.GA9091@hu-pkondeti-hyd.qualcomm.com>
 <20220915062027.GA14713@hu-pkondeti-hyd.qualcomm.com>
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 <CAJuCfpFr3JfwkWbDqkU=NUJbCYuCWGySwNusMCdmS3z95WD2AQ@mail.gmail.com>
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On 2022/10/9 20:41, Chengming Zhou wrote:
> Hello,
> 
> I just saw these emails, sorry for late.
> 
> On 2022/10/6 00:32, Suren Baghdasaryan wrote:
>> On Sun, Oct 2, 2022 at 11:11 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>>
>>> On Fri, Sep 16, 2022 at 10:45 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>>>
>>>> On Wed, Sep 14, 2022 at 11:20 PM Pavan Kondeti
>>>> <quic_pkondeti@quicinc.com> wrote:
>>>>>
>>>>> On Tue, Sep 13, 2022 at 07:38:17PM +0530, Pavan Kondeti wrote:
>>>>>> Hi
>>>>>>
>>>>>> The fact that psi_avgs_work()->collect_percpu_times()->get_recent_times()
>>>>>> run from a kworker thread, PSI_NONIDLE condition would be observed as
>>>>>> there is a RUNNING task. So we would always end up re-arming the work.
>>>>>>
>>>>>> If the work is re-armed from the psi_avgs_work() it self, the backing off
>>>>>> logic in psi_task_change() (will be moved to psi_task_switch soon) can't
>>>>>> help. The work is already scheduled. so we don't do anything there.
>>>>
>>>> Hi Pavan,
>>>> Thanks for reporting the issue. IIRC [1] was meant to fix exactly this
>>>> issue. At the time it was written I tested it and it seemed to work.
>>>> Maybe I missed something or some other change introduced afterwards
>>>> affected the shutoff logic. I'll take a closer look next week when I'm
>>>> back at my computer and will consult with Johannes.
>>>
>>> Sorry for the delay. I had some time to look into this and test psi
>>> shutoff on my device and I think you are right. The patch I mentioned
>>> prevents new psi_avgs_work from being scheduled when the only non-idle
>>> task is psi_avgs_work itself, however the regular 2sec averaging work
>>> will still go on. I think we could record the fact that the only
>>> active task is psi_avgs_work in record_times() using a new
>>> psi_group_cpu.state_mask flag and then prevent psi_avgs_work() from
>>> rescheduling itself if that flag is set for all non-idle cpus. I'll
>>> test this approach and will post a patch for review if that works.
>>
>> Hi Pavan,
>> Testing PSI shutoff on Android proved more difficult than I expected.
>> Lots of tasks to silence and I keep encountering new ones.
>> The approach I was thinking about is something like this:
>>
>> ---
>>  include/linux/psi_types.h |  3 +++
>>  kernel/sched/psi.c        | 12 +++++++++---
>>  2 files changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
>> index c7fe7c089718..8d936f22cb5b 100644
>> --- a/include/linux/psi_types.h
>> +++ b/include/linux/psi_types.h
>> @@ -68,6 +68,9 @@ enum psi_states {
>>          NR_PSI_STATES = 7,
>>  };
>>
>> +/* state_mask flag to keep re-arming averaging work */
>> +#define PSI_STATE_WAKE_CLOCK        (1 << NR_PSI_STATES)
>> +
>>  enum psi_aggregators {
>>          PSI_AVGS = 0,
>>          PSI_POLL,
>> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
>> index ecb4b4ff4ce0..dd62ad28bacd 100644
>> --- a/kernel/sched/psi.c
>> +++ b/kernel/sched/psi.c
>> @@ -278,6 +278,7 @@ static void get_recent_times(struct psi_group
>> *group, int cpu,
>>                  if (delta)
>>                          *pchanged_states |= (1 << s);
>>          }
>> +        *pchanged_states |= (state_mask & PSI_STATE_WAKE_CLOCK);
> 
> If the avgs_work kworker is running on this CPU, it will still see
> PSI_STATE_WAKE_CLOCK set in state_mask? So the work will be re-armed?
> 
> Maybe I missed something... but I have another different idea which
> I want to implement later only for discussion.

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index ee2ecc081422..f322e8fd8d41 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -241,11 +241,13 @@ static void get_recent_times(struct psi_group *group, int cpu,
                             enum psi_aggregators aggregator, u32 *times,
                             u32 *pchanged_states)
 {
+       int current_cpu = raw_smp_processor_id();
        struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
        u64 now, state_start;
        enum psi_states s;
        unsigned int seq;
        u32 state_mask;
+       bool only_avgs_work = false;

        *pchanged_states = 0;

@@ -256,6 +258,14 @@ 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;
+               /*
+                * This CPU has only avgs_work kworker running, snapshot the
+                * newest times then don't need to re-arm work for this groupc.
+                * Normally this kworker will sleep soon and won't
+                * wake_clock in psi_group_change().
+                */
+               if (current_cpu == cpu && groupc->tasks[NR_RUNNING] == 1)
+                       only_avgs_work = true;
        } while (read_seqcount_retry(&groupc->seq, seq));

        /* Calculate state time deltas against the previous snapshot */
@@ -280,6 +290,10 @@ 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);
 }

 static void calc_avgs(unsigned long avg[3], int missed_periods,


> 
> Thanks.
> 
>>  }
>>
>>  static void calc_avgs(unsigned long avg[3], int missed_periods,
>> @@ -413,7 +414,7 @@ static void psi_avgs_work(struct work_struct *work)
>>          struct delayed_work *dwork;
>>          struct psi_group *group;
>>          u32 changed_states;
>> -        bool nonidle;
>> +        bool wake_clock;
>>          u64 now;
>>
>>          dwork = to_delayed_work(work);
>> @@ -424,7 +425,7 @@ 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);
>> +        wake_clock = changed_states & PSI_STATE_WAKE_CLOCK;
>>          /*
>>           * If there is task activity, periodically fold the per-cpu
>>           * times and feed samples into the running averages. If things
>> @@ -435,7 +436,7 @@ 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 (wake_clock) {
>>                  schedule_delayed_work(dwork, nsecs_to_jiffies(
>>                                  group->avg_next_update - now) + 1);
>>          }
>> @@ -742,6 +743,11 @@ static void psi_group_change(struct psi_group
>> *group, int cpu,
>>          if (unlikely(groupc->tasks[NR_ONCPU] && cpu_curr(cpu)->in_memstall))
>>                  state_mask |= (1 << PSI_MEM_FULL);
>>
>> +        if (wake_clock || test_state(groupc->tasks, PSI_NONIDLE)) {
>> +                /* psi_avgs_work was not the only task on the CPU */
>> +                state_mask |= PSI_STATE_WAKE_CLOCK;
>> +        }
>> +
>>          groupc->state_mask = state_mask;
>>
>>          write_seqcount_end(&groupc->seq);