linux-kernel - Re: [PATCH v4 2/2] ksm: provide support to use deferrable timers for scanner thread

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Message-ID: <alpine.LSU.2.11.1409110609320.2465@eggly.anvils>
Date:	Thu, 11 Sep 2014 06:25:09 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Chintan Pandya <cpandya@...eaurora.org>
cc:	Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 2/2] ksm: provide support to use deferrable timers
 for scanner thread

On Thu, 11 Sep 2014, Chintan Pandya wrote:

> I don't mean to divert the thread too much. But just one suggestion offered
> by Harshad.
> 
> Why can't we stop invoking more of a KSM scanner thread when we are
> saturating from savings ? But again, to check whether savings are saturated
> or not, we may still want to rely upon timers and we have to wake the CPUs up
> from IDLE state.

I agree that it should make sense for KSM to slow down when it sees it's
making no progress (though that would depart from the pages_to_scan and
sleep_millisecs prescription - perhaps could be tied to sleep_millisecs 0).

But not stop.  That's the problem we're mainly concerned with here:
to save power we need it to stop, but then how to wake up, without
putting nasty hooks in hot paths for a minority interest?
I don't see an answer to that above.

Hugh
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