linux-kernel - Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

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Message-ID: <20150330151431.GA23123@twins.programming.kicks-ass.net>
Date:	Mon, 30 Mar 2015 17:14:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>, hannes@...xchg.org,
	Christoph Lameter <cl@...ux.com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	vinmenon@...eaurora.org, shashim@...eaurora.org, mgorman@...e.de,
	dave@...olabs.net, koct9i@...il.com,
	Linux Memory Management List <linux-mm@...ck.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

On Mon, Mar 30, 2015 at 05:08:18PM +0200, Michal Hocko wrote:
> On Sat 28-03-15 10:53:22, Peter Zijlstra wrote:
> [...]
> > Alternatively the thing hocko suggests is an utter fail too. You cannot
> > stuff that into hardirq context, that's insane.
> 
> I guess you are referring to
> http://article.gmane.org/gmane.linux.kernel.mm/127569, right?
> 
> Why cannot we do something like refresh_cpu_vm_stats from the IRQ
> context?  Especially the first zone stat part.

Big machines have big zone counts. There are machines with >200 nodes.
Although with the current trend of bigger nodes, the number of nodes
seems to come down as well. Still.

> The per-cpu pagesets is
> more costly and it would need a special treatment, alright. A simple
> way would be to splice the lists from the per-cpu context and then free
> those pages from the kthread context.
> 
> I am still wondering why those two things were squashed into a single
> place. Why kswapd is not doing the pcp cleanup?

Probably because they could be. The problem with kswapd is that its per
node, not per cpu.
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