linux-kernel - Re: [patch 3/5] mm: vmstat: optionally flush per-CPU vmstat counters on return to userspace

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Message-ID: <20210712103023.GA20945@fuller.cnet>
Date:   Mon, 12 Jul 2021 07:30:23 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Christoph Lameter <cl@...two.de>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Nitesh Lal <nilal@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolas Saenz <nsaenzju@...hat.com>
Subject: Re: [patch 3/5] mm: vmstat: optionally flush per-CPU vmstat counters
 on return to userspace

On Mon, Jul 12, 2021 at 11:05:58AM +0200, Christoph Lameter wrote:
> On Fri, 9 Jul 2021, Marcelo Tosatti wrote:
> 
> > +
> > +	if (!static_branch_unlikely(&vmstat_sync_enabled))
> > +		return;
> > +
> > +	cpu = smp_processor_id();
> > +
> > +	if (housekeeping_cpu(cpu, HK_FLAG_QUIESCE_URET))
> > +		return;
> > +
> > +	per_cpu(vmstat_dirty, smp_processor_id()) = true;
> > +}
> 
> And you are going to insert this into all the performance critical VM
> statistics handling. Inline?

Yes, this is what the patch below is supposed to do (maybe it missed
some statistics?).

The alternative would be some equivalent of need_update on return to
userspace (for all system call returns) (when the HK_FLAG_QUIESCE_URET 
flag is enabled).

> And why do you need to do such things as to determine the processor? At
> mininum do this using this cpu operations like the vmstat functions
> currently do.

OK, will do that and resend.

> And, lucky us, now we also have
> more issues why we should disable preemption etc etc while handling vm
> counters.