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

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Date:	Fri, 27 Mar 2015 10:30:23 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	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,
	Michal Hocko <mhocko@...e.cz>, 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 Fri, Mar 27, 2015 at 10:16:13AM +0100, Peter Zijlstra wrote:
> On Fri, Mar 27, 2015 at 10:19:54AM +0530, Viresh Kumar wrote:
> > On 27 March 2015 at 01:48, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > Shouldn't this be viewed as a shortcoming of the core timer code?
> > 
> > Yeah, it is. Some (not so pretty) solutions were tried earlier to fix that, but
> > they are rejected for obviously reasons [1].
> > 
> > > vmstat_shepherd() is merely rescheduling itself with
> > > schedule_delayed_work().  That's a dead bog simple operation and if
> > > it's producing suboptimal behaviour then we shouldn't be fixing it with
> > > elaborate workarounds in the caller?
> > 
> > I understand that, and that's why I sent it as an RFC to get the discussion
> > started. Does anyone else have got another (acceptable) idea to get this
> > resolved ?
> 
> So the issue seems to be that we need base->running_timer in order to
> tell if a callback is running, right?
> 
> We could align the base on 8 bytes to gain an extra bit in the pointer
> and use that bit to indicate the running state. Then these sites can
> spin on that bit while we can change the actual base pointer.

Even though tvec_base has ____cacheline_aligned stuck on, most are
allocated using kzalloc_node() which does not actually respect that but
already guarantees a minimum u64 alignment, so I think we can use that
third bit without too much magic.
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