[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]

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?

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