Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

[Andrew Morton]

On Thu, 26 Mar 2015 11:09:01 +0530 Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:

> A delayed work to schedule vmstat_shepherd() is queued at periodic intervals for
> internal working of vmstat core. This work and its timer end up waking an idle
> cpu sometimes, as this always stays on CPU0.
> 
> Because we re-queue the work from its handler, idle_cpu() returns false and so
> the timer (used by delayed work) never migrates to any other CPU.
> 
> This may not be the desired behavior always as waking up an idle CPU to queue
> work on few other CPUs isn't good from power-consumption point of view.
> 
> In order to avoid waking up an idle core, we can replace schedule_delayed_work()
> with a normal work plus a separate timer. The timer handler will then queue the
> work after re-arming the timer. If the CPU was idle before the timer fired,
> idle_cpu() will mostly return true and the next timer shall be migrated to a
> non-idle CPU.
> 
> But the timer core has a limitation, when the timer is re-armed from its
> handler, timer core disables migration of that timer to other cores. Details of
> that limitation are present in kernel/time/timer.c:__mod_timer() routine.
> 
> Another simple yet effective solution can be to keep two timers with same
> handler and keep toggling between them, so that the above limitation doesn't
> hold true anymore.
> 
> This patch replaces schedule_delayed_work() with schedule_work() plus two
> timers. After this, it was seen that the timer and its do get migrated to other
> non-idle CPUs, when the local cpu is idle.

Shouldn't this be viewed as a shortcoming of the core timer code? 

vmstat_shepherd() is merely rescheduling itself with
schedule_delayed_work().  That's a dead bog simple operation and if
it's producing suboptimal behaviour then we shouldn't be fixing it with
elaborate workarounds in the caller?

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