[PATCH] vmstat: make vmstat_update deferrable (was: Re: [PATCH] mm, vmstat: make quiet_vmstat lighter) path cycle muncher (vmstat: make vmstat_updater deferrable) again and shut down on idle)

[Michal Hocko <mhocko@kernel.org>]

On Thu 28-01-16 16:31:11, Michal Hocko wrote:
> On Wed 27-01-16 12:26:16, Christoph Lameter wrote:
> > On Wed, 27 Jan 2016, Michal Hocko wrote:
> [...]
> > > +void quiet_vmstat(void)
> > > +{
> > > +	if (system_state != SYSTEM_RUNNING)
> > > +		return;
> > > +
> > > +	/*
> > > +	 * If we are already in hands of the shepherd then there
> > > +	 * is nothing for us to do here.
> > > +	 */
> > > +	if (cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
> > > +		return;
> > > +
> > > +	if (!need_update(smp_processor_id()))
> > > +		return;
> > > +
> > > +	/*
> > > +	 * Just refresh counters and do not care about the pending delayed
> > > +	 * vmstat_update. It doesn't fire that often to matter and canceling
> > > +	 * it would be too expensive from this path.
> > > +	 * vmstat_shepherd will take care about that for us.
> > > +	 */
> > > +	refresh_cpu_vm_stats(false);
> > > +}
> > 
> > The problem here is that there will be an additional tick generated on
> > idle. This is an issue for power because now the processor has to
> > needlessly wake up again, do tick processing etc  just to effectively do a
> > cancel_delayed_work().
> 
> Thinking about it some more, making vmstat_update deferrable should help
> to not interrupt idle no?

With the full changelog
>From 89858bf594c020f065b4f415a12d75bee64a7954 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Thu, 28 Jan 2016 16:31:43 +0100
Subject: [PATCH] vmstat: make vmstat_update deferrable

0eb77e988032 ("vmstat: make vmstat_updater deferrable again and shut
down on idle") made vmstat_shepherd deferrable. vmstat_update itself
is still using standard timer which might interrupt idle task. This
is possible because "mm, vmstat: make quiet_vmstat lighter" removed
cancel_delayed_work from the quiet_vmstat. Change vmstat_work to
use DEFERRABLE_WORK to prevent from pointless wakeups from the idle
context.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/vmstat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index eb30bf45bd55..69537d2be6f6 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1512,7 +1512,7 @@ static void __init start_shepherd_timer(void)
 	int cpu;
 
 	for_each_possible_cpu(cpu)
-		INIT_DELAYED_WORK(per_cpu_ptr(&vmstat_work, cpu),
+		INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu),
 			vmstat_update);
 
 	if (!alloc_cpumask_var(&cpu_stat_off, GFP_KERNEL))
-- 
2.7.0.rc3


-- 
Michal Hocko
SUSE Labs