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Message-ID: <d4918ecc-e1a9-7c16-fdf1-10cac60cf490@bytedance.com>
Date: Thu, 13 Oct 2022 10:23:04 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: hannes@...xchg.org, quic_pkondeti@...cinc.com,
peterz@...radead.org, quic_charante@...cinc.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work()
On 2022/10/13 02:24, Suren Baghdasaryan wrote:
> On Tue, Oct 11, 2022 at 7:11 PM Chengming Zhou
> <zhouchengming@...edance.com> wrote:
>>
>> On 2022/10/12 01:00, Suren Baghdasaryan wrote:
>>> On Mon, Oct 10, 2022 at 5:07 PM Chengming Zhou
>>> <zhouchengming@...edance.com> wrote:
>>>>
>>>> Hello,
>>>>
>>>> On 2022/10/11 05:21, Suren Baghdasaryan wrote:
>>>>> On Mon, Oct 10, 2022 at 3:42 AM Chengming Zhou
>>>>> <zhouchengming@...edance.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@...cinc.com>
>>>>>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.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));
Ok, I get what you mean: for other CPUs, reschedule == PSI_NONIDLE,
for this current CPU, reschedule is calculated from groupc->tasks[].
Then we use PSI_STATE_RESCHEDULE in psi_avgs_work(), don't use PSI_NONIDLE,
so PSI_NONIDLE become useless in changed_states. Right?
Well, I'm not sure this is easier than just using PSI_NONIDLE. But anyway,
I will try this way in the next revision.
Thanks.
>
> /* 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.
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