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SubjectRe: [PATCH] mm, vmstat: make quiet_vmstat lighter (was: Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable) again and shut down on idle)
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?
---
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))
--
Michal Hocko
SUSE Labs
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