Messages in this thread | | | From | Suren Baghdasaryan <> | Date | Tue, 11 Oct 2022 10:00:48 -0700 | Subject | Re: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work() |
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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.
> > > > >> } 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. Thanks, Suren.
> > 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! >
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