linux-kernel - Re: [PATCH v13 1/6] mm/vmstat: Add CPU-specific variable to track a vmstat discrepancy

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Message-ID: <Y71WGeT2+6s4ewfv@lothringen>
Date:   Tue, 10 Jan 2023 13:12:09 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Christoph Lameter <cl@...two.de>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>, atomlin@...mlin.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 v13 1/6] mm/vmstat: Add CPU-specific variable to track a
 vmstat discrepancy

On Tue, Jan 10, 2023 at 12:58:32PM +0100, Christoph Lameter wrote:
> On Thu, 5 Jan 2023, Marcelo Tosatti wrote:
> 
> > +static inline void vmstat_mark_dirty(void)
> > +{
> > +	this_cpu_write(vmstat_dirty, true);
> > +}
> 
> this_cpu_write() is intended for an per cpu atomic context. You are not
> using it in that way. The processor may have changed before or after and
> thus vmstat_dirty for another CPU may  have been marked dirty.
> 
> I guess this would have to be called __vmstat_mark_dirty() and be using
> __this_cpu_write(*) with a requirement that preemption be disabled before
> using this function.

You're right. So this patchset also arranges for these vmstat functions to be
called with preemption disabled. I'm converting the this_cpu operations
to __this_cpu versions to make sure of that. And I believe the __this_cpu
WARN if preemptible().


> 
> > +static inline void vmstat_clear_dirty(void)
> > +{
> > +	this_cpu_write(vmstat_dirty, false);
> > +}
> 
> Same
> 
> > +static inline bool is_vmstat_dirty(void)
> > +{
> > +	return this_cpu_read(vmstat_dirty);
> > +}
> 
> This function would only work correctly if preemption is disabled.
> Otherwise the processor may change.

Indeed that should apply as __this_cpu_read() as well.

Thanks!