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

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Date:	Thu, 11 Sep 2014 13:31:13 +0530
From:	Chintan Pandya <cpandya@...eaurora.org>
To:	Hugh Dickins <hughd@...gle.com>
CC:	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

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.

>> here. Can't we create a new (timer) infrastructure that does the right
>> thing? Surely this isn't the only such case.
>
> A sleep-walking timer, that goes to sleep in one bed, but may wake in
> another; and defers while beds are empty?  I'd be happy to try using
> that for KSM if it already existed, and no doubt Chintan would too

This is interesting for sure :)

>
> But I don't think KSM presents a very good case for developing it.
> I think KSM's use of a sleep_millisecs timer is really just an apology
> for the amount of often wasted work that it does, and dates from before
> we niced it down 5.  I prefer the idea of a KSM which waits on activity
> amongst the restricted set of tasks it is tracking: as this patch tries.
>
> But my preference may be naive: doing lots of unnecessary work doesn't
> matter as much as waking cpus from deep sleep.

This is exactly the preference we are looking for. But yes, cannot be 
generalized for all.

>
>>
>> I know both RCU and some NOHZ_FULL muck already track when the system is
>> completely idle. This is yet another case of that.
>
> Hugh


-- 
Chintan Pandya

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