Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose number of tasks running on cpu

From: Tim Chen
Date: Tue Jul 15 2014 - 14:41:52 EST

On Tue, 2014-07-15 at 14:59 +0200, Thomas Gleixner wrote:
> On Tue, 15 Jul 2014, Peter Zijlstra wrote:
> > On Tue, Jul 15, 2014 at 11:50:45AM +0200, Peter Zijlstra wrote:
> > > So you already have an idle notifier (which is x86 only, we should fix
> > > that I suppose), and you then double check there really isn't anything
> > > else running.
> >
> > Note that we've already done a large part of the expense of going idle
> > by the time we call that idle notifier -- in specific, we've
> > reprogrammed the clock to stop the tick.
> >
> > Its really wasteful to then generate work again, which means we have to
> > again reprogram the clock etc.
> Doing anything which is not related to idle itself in the idle
> notifier is just plain wrong.

I don't like the kicking the multi-buffer job flush using idle_notifier
path either. I'll try another version of the patch by doing this in the
multi-buffer job handler path.

> If that stuff wants to utilize idle slots, we really need to come up
> with a generic and general solution. Otherwise we'll grow those warts
> all over the architecture space, with slightly different ways of
> wreckaging the world an some more.
> This whole attidute of people thinking that they need their own
> specialized scheduling around the real scheduler is a PITA. All this
> stuff is just damanging any sensible approach of power saving, load
> balancing, etc.
> What we really want is infrastructure, which allows the scheduler to
> actively query the async work situation and based on the results
> actively decide when to process it and where.

I agree with you. It will be great if we have such infrastructure.



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