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

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Message-ID: <20150330135948.GY23123@twins.programming.kicks-ass.net>
Date:	Mon, 30 Mar 2015 15:59:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <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,
	Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>,
	dave@...olabs.net, Konstantin Khlebnikov <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 06:44:22PM +0530, Viresh Kumar wrote:
> On 30 March 2015 at 18:17, Peter Zijlstra <peterz@...radead.org> wrote:
> > No, I means something else with that. We can remove the
> > tvec_base::running_timer field. Everything that uses that can use
> > tbase_running() AFAICT.
> 
> Okay, there is one instance which still needs it.
> 
> migrate_timers():
> 
>         BUG_ON(old_base->running_timer);
> 
> What I wasn't sure about it is if we get can drop this statement or not.
> If we decide not to drop it, then we can convert running_timer into a bool.

Yeah, so that _should_ not trigger (obviously), and while I agree with
the sentiment of sanity checks, I'm not sure its worth keeping that
variable around just for that.

Anyway, while I'm looking at struct tvec_base I notice the cpu member
should be second after the lock, that'll save 8 bytes on the structure
on 64bit machines.
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