Messages in this thread Patch in this message | | | Date | Thu, 13 Oct 2022 19:06:55 +0800 | Subject | Re: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work() | From | Chengming Zhou <> |
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On 2022/10/13 02:24, Suren Baghdasaryan wrote: > 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)); We can't use times[] here because it will incorporate currently active states on the CPU.
Should I still need to copy groupc->tasks[] out for the current_cpu as you suggested before?
> > /* 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;
Is this ok?
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index 4bd3913dd176..0cbcbf89a934 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 reschedule; u64 now, state_start; enum psi_states s; unsigned int seq; @@ -256,6 +258,10 @@ 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) + reschedule = groupc->tasks[NR_RUNNING] + + groupc->tasks[NR_IOWAIT] + + groupc->tasks[NR_MEMSTALL] > 1; } while (read_seqcount_retry(&groupc->seq, seq));
/* Calculate state time deltas against the previous snapshot */ @@ -280,6 +286,12 @@ static void get_recent_times(struct psi_group *group, int cpu, if (delta) *pchanged_states |= (1 << s); } + + if (cpu != current_cpu) + reschedule = *pchanged_states & (1 << PSI_NONIDLE); + + if (reschedule) + *pchanged_states |= (1 << PSI_STATE_RESCHEDULE); }
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