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Date:   Fri, 16 Dec 2022 13:16:09 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     atomlin@redhat.com, cl@linux.com, tglx@linutronix.de,
        mingo@kernel.org, peterz@infradead.org, pauld@redhat.com,
        neelx@redhat.com, oleksandr@natalenko.name,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v9 3/5] mm/vmstat: manage per-CPU stats from CPU context
 when NOHZ full
Message-ID: <Y5yZyd0gd7M3M2+j@tpad>
References: <20221206161826.698593151@redhat.com>
 <20221206162416.474800121@redhat.com>
 <20221214133302.GA1931356@lothringen>
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On Wed, Dec 14, 2022 at 02:33:02PM +0100, Frederic Weisbecker wrote:
> 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?

Don't think the mistake is an issue. In case of a
queue_delayed_work_on being called after cancel_delayed_work_sync,
either vmstat_update executes on the local CPU, or on a
different CPU (after the bound kworkers have been moved).

Each case is fine (see vmstat_update).

> > +		}
> > +	}
> >  	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.

Fixed.