linux-kernel - Re: [External] [tip: sched/core] sched/psi: Fix avgs_work re-arm in psi_avgs

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Message-ID: <f990a324-e28e-6de1-acb0-ba764808a56a@bytedance.com>
Date:   Fri, 28 Oct 2022 14:50:09 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     linux-tip-commits@...r.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc:     Pavan Kondeti <quic_pkondeti@...cinc.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        "surenb@...gle.com" <surenb@...gle.com>
Subject: Re: [External] [tip: sched/core] sched/psi: Fix avgs_work re-arm in
 psi_avgs_work()

Hello,

Thanks for picking this up. There is a newer version which has been acked:
https://lore.kernel.org/all/20221014110551.22695-1-zhouchengming@bytedance.com/

As well another PSI patch that has been acked by Johannes:
https://lore.kernel.org/all/20220926081931.45420-1-zhouchengming@bytedance.com/

Thanks!


On 2022/10/28 14:42, tip-bot2 for Chengming Zhou wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     7d89d7bb921c5ae5a428df282e64ee5692e26fe0
> Gitweb:        https://git.kernel.org/tip/7d89d7bb921c5ae5a428df282e64ee5692e26fe0
> Author:        Chengming Zhou <zhouchengming@...edance.com>
> AuthorDate:    Mon, 10 Oct 2022 18:42:06 +08:00
> Committer:     Peter Zijlstra <peterz@...radead.org>
> CommitterDate: Thu, 27 Oct 2022 11:01:23 +02:00
> 
> sched/psi: Fix avgs_work re-arm in psi_avgs_work()
> 
> 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>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lore.kernel.org/r/20221010104206.12184-1-zhouchengming@bytedance.com
> ---
>  kernel/sched/psi.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index ee2ecc0..f4cdf6f 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;
>  	} 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);
>  }
>  
>  static void calc_avgs(unsigned long avg[3], int missed_periods,