Messages in this thread Patch in this message | | | Date | Tue, 11 Oct 2022 08:07:11 +0800 | Subject | Re: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work() | From | Chengming Zhou <> |
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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!
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