Messages in this thread Patch in this message | | | Date | Wed, 12 Oct 2022 10:10:59 +0800 | Subject | Re: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work() | From | Chengming Zhou <> |
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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().
Thanks.
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