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

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Date:	Wed, 27 Jan 2016 04:12:27 +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)

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) 

> 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;