linux-kernel - Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)

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Message-ID: <1453868124.3622.31.camel@gmail.com>
Date:	Wed, 27 Jan 2016 05:15:24 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable
 again and shut down on idle)

On Wed, 2016-01-27 at 04:12 +0100, Mike Galbraith wrote:
> Good morning,
> 
> On Tue, 2016-01-26 at 13:20 -0600, Christoph Lameter wrote:
> > On Tue, 26 Jan 2016, Mike Galbraith wrote:
> > 
> > > > The system going idle is a fastpath. Mind boogling.
> > > 
> > > Hohum, noted.  Now what about those cycles, and the sleeping lock you
> > > injected for -rt?
> > 
> > Since we (the NOHZ people) care mostly about NOHZ then lets restrict
> > that to the NOHZ mode. Then it should not affect your load.
> 
> Tons of folks do have NO_HZ enabled (including me).  Isn't there a spot
> somewhere in NO_HZ_FULL code where it can take up residence?  (one with
> a tad lower maximum call frequency would be good, a nohz_full cpu isn't
> necessarily being used for pure compute exclusively)

I forgot to mention that the spot you picked is called with irqs
disabled.

> > Subject: Move quiet_vmstat() to NOHZ code
> > 
> > quiet_vmstat() seems to cause regressions for some load because
> > the cpu going idle is a "fastpath". Mind boogling. Strange claim.
> > If the system goes idle then it has nothing to do after all

Hm, seems I also forgot to say "Hohum, noted..." again.

	-Mike