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Date:   Tue, 11 Oct 2022 08:07:11 +0800
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Subject: Re: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work()
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     hannes@cmpxchg.org, quic_pkondeti@quicinc.com,
        peterz@infradead.org, quic_charante@quicinc.com,
        linux-kernel@vger.kernel.org
References: <20220913140817.GA9091@hu-pkondeti-hyd.qualcomm.com>
 <20221010104206.12184-1-zhouchengming@bytedance.com>
 <CAJuCfpF7Z+CYhk-f_aaDTE232+m9z_n-QfjGfdLje7QrX9bFtw@mail.gmail.com>
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From:   Chengming Zhou <zhouchengming@bytedance.com>
In-Reply-To: <CAJuCfpF7Z+CYhk-f_aaDTE232+m9z_n-QfjGfdLje7QrX9bFtw@mail.gmail.com>
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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.

> 
>>         } 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.

I'm not sure if it's better to clear here, maybe we can add a comment in get_recent_times()
when do PSI_NONIDLE clear?

Thanks!