linux-kernel - Re: [PATCH v9 3/5] mm/vmstat: manage per-CPU stats from CPU context when NOHZ full

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Message-ID: <20221214133302.GA1931356@lothringen>
Date:   Wed, 14 Dec 2022 14:33:02 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     atomlin@...hat.com, cl@...ux.com, tglx@...utronix.de,
        mingo@...nel.org, peterz@...radead.org, pauld@...hat.com,
        neelx@...hat.com, oleksandr@...alenko.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v9 3/5] mm/vmstat: manage per-CPU stats from CPU context
 when NOHZ full

On Tue, Dec 06, 2022 at 01:18:29PM -0300, Marcelo Tosatti wrote:
>  static inline void vmstat_mark_dirty(void)
>  {
> +	int cpu = smp_processor_id();
> +
> +	if (tick_nohz_full_cpu(cpu) && !this_cpu_read(vmstat_dirty)) {
> +		struct delayed_work *dw;
> +
> +		dw = &per_cpu(vmstat_work, cpu);
> +		if (!delayed_work_pending(dw)) {
> +			unsigned long delay;
> +
> +			delay = round_jiffies_relative(sysctl_stat_interval);
> +			queue_delayed_work_on(cpu, mm_percpu_wq, dw, delay);

Currently the vmstat_work is flushed on cpu_hotplug (CPUHP_AP_ONLINE_DYN).
vmstat_shepherd makes sure to not rearm it afterward. But now it looks
possible for the above to do that mistake?

> +		}
> +	}
>  	this_cpu_write(vmstat_dirty, true);
>  }
> @@ -2009,6 +2028,10 @@ static void vmstat_shepherd(struct work_
>  	for_each_online_cpu(cpu) {
>  		struct delayed_work *dw = &per_cpu(vmstat_work, cpu);
>  
> +		/* NOHZ full CPUs manage their own vmstat flushing */
> +		if (tick_nohz_full_cpu(smp_processor_id()))

It should be the remote CPU instead of the current one.

Thanks.

> +			continue;
> +
>  		if (!delayed_work_pending(dw) && per_cpu(vmstat_dirty, cpu))
>  			queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
>