linux-kernel - Re: [PATCH v2] mm: vmscan: fix the page state calculation in too_many

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Date:	Mon, 26 Jan 2015 12:35:00 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Vinayak Menon <vinmenon@...eaurora.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	hannes@...xchg.org, vdavydov@...allels.com, mgorman@...e.de,
	minchan@...nel.org
Subject: Re: [PATCH v2] mm: vmscan: fix the page state calculation in
 too_many_isolated

On Mon, 26 Jan 2015, Michal Hocko wrote:

> > Please do not run the vmstat_updates concurrently. They update shared
> > cachelines and therefore can cause bouncing cachelines if run concurrently
> > on multiple cpus.
>
> Would you preffer to call smp_call_function_single on each CPU
> which needs an update? That would make vmstat_shepherd slower but that
> is not a big deal, is it?

Run it from the timer interrupt as usual from a work request? Those are
staggered.

> Anyway I am wondering whether the cache line bouncing between
> vmstat_update instances is a big deal in the real life. Updating shared
> counters whould bounce with many CPUs but this is an operation which is
> not done often. Also all the CPUs would have update the same counters
> all the time and I am not sure this happens that often. Do you have a
> load where this would be measurable?

Concurrent page faults update lots of counters concurrently. But will
those trigger the smp_call_function?
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