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

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Date:	Tue, 27 Jan 2015 11:52:42 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Christoph Lameter <cl@...ux.com>
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 26-01-15 12:35:00, Christoph Lameter wrote:
> 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.

I am not following. The idea was to run vmstat_shepherd in a kernel
thread and waking up as per defined timeout and then check need_update
for each CPU and call smp_call_function_single to refresh the timer
rather than building a mask and then calling sm_call_function_many to
reduce paralel contention on the shared counters.

> > 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.

True

> But will those trigger the smp_call_function?

The smp_call_function was meant to be called only from the
vmstat_shepherd context which does happen "rarely". Or am I missing your
point here?

-- 
Michal Hocko
SUSE Labs
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