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

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Message-ID: <alpine.DEB.2.20.1601271028001.14468@east.gentwo.org>
Date:	Wed, 27 Jan 2016 10:28:48 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Michal Hocko <mhocko@...nel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	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, 27 Jan 2016, Mike Galbraith wrote:

> > 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)

Frederic, any idea on where to put quiet_vmstat()?


> > 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
> >
> > But anyways if we shift the quiet_vmstat() into the NOHZ logic
> > when it stops the tick then it will only affect those cores that
> > are setup for NOHZ mode. That is where we want this processing
> > after all to ensure that the OS keeps itself off those cores.
> >
> > Signed-off-by: Christoph Lameter <cl@...ux.com>
> >
> >
> > Index: linux/kernel/sched/idle.c
> > ===================================================================
> > --- linux.orig/kernel/sched/idle.c
> > +++ linux/kernel/sched/idle.c
> > @@ -213,7 +213,6 @@ static void cpu_idle_loop(void)
> >  		 */
> >
> >  		__current_set_polling();
> > -		quiet_vmstat();
> >  		tick_nohz_idle_enter();
> >
> >  		while (!need_resched()) {
> > Index: linux/kernel/time/tick-sched.c
> > ===================================================================
> > --- linux.orig/kernel/time/tick-sched.c
> > +++ linux/kernel/time/tick-sched.c
> > @@ -811,6 +811,7 @@ static void __tick_nohz_idle_enter(struc
> >  		ts->idle_calls++;
> >
> >  		expires = tick_nohz_stop_sched_tick(ts, now, cpu);
> > +		quiet_vmstat();
> >  		if (expires.tv64 > 0LL) {
> >  			ts->idle_sleeps++;
> >  			ts->idle_expires = expires;
>