linux-kernel - Re: [PATCH v12 2/6] mm/vmstat: Use vmstat_dirty to track CPU-specific vmstat discrepancies

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Date:   Sat, 31 Dec 2022 21:38:04 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     atomlin@...mlin.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 v12 2/6] mm/vmstat: Use vmstat_dirty to track
 CPU-specific vmstat discrepancies

On Fri, Dec 30, 2022 at 02:21:32PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 27, 2022 at 09:11:39AM -0300, Marcelo Tosatti wrote:
> > @@ -606,6 +608,7 @@ static inline void mod_zone_state(struct
> >  
> >  	if (z)
> >  		zone_page_state_add(z, zone, item);
> > +	vmstat_mark_dirty();
> >  }
> >  
> >  void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
> > @@ -674,6 +677,7 @@ static inline void mod_node_state(struct
> >  
> >  	if (z)
> >  		node_page_state_add(z, pgdat, item);
> > +	vmstat_mark_dirty();
> 
> Looking at this further, about the two above chunks, there is a risk to
> mark the wrong CPU dirty because those functions are preemptible and rely
> on this_cpu_cmpxchg() to deal with preemption.
> 
> Thanks.

Hi Frederic,

Yes, good catch: if the CPU is preempted after this_cpu_cmpxchg(),
but before vmstat_mark_dirty, then one ends up with a CPU with
per-CPU vm counters dirty and the per-CPU vmstat dirty bit unset.

This could cause a CPU to remain with the per-CPU vm counters dirty
for longer than sysctl_stat_interval.

Should move vmstat_mark_dirty() above "if (z)", then do
preempt_disable() on function entry and preempt_enable()
after vmstat_mark_dirty. Luckily preempt_disable()/preempt_enable()
is much cheaper than local_irq_disable()/local_irq_enable().