Re: [RFC PATCH 15/15] nohz_task: Procfs interface

[Avi Kivity <avi@redhat.com>]

On 12/21/2010 11:08 PM, Frederic Weisbecker wrote:
> On Tue, Dec 21, 2010 at 08:17:33PM +0200, Avi Kivity wrote:
> >  On 12/21/2010 07:05 PM, Frederic Weisbecker wrote:
> >  >>
> >  >>   For an example of a per-cpu flag that is checked on every exit with
> >  >>   zero additional overhead on the flag clear case, look at
> >  >>   TIF_USER_RETURN_NOTIFY.
> >  >
> >  >Right, but the problem is actually that if we want to automate the nohz
> >  >attribute on every tasks, then you need you have this flag set for
> >  >all of these threads.
> >  >
> >  >No problem with that, but if nobody wants the nohz attribute, we don't
> >  >need to force that slow path.
> >
> >  When the scheduler detects the task is all alone, it sets the flag;
> >  when it blocks, or if another task joins, it drops the flag (at most
> >  one task per cpu has the flag set).
> >
> >  Does that work?
>
> Makes sense. And that integrates well with Peter's idea of creating a
> new cpuset attribute for the nohz tasks.
>
> But instead of making this detection from the scheduler, I think this
> should be done from the tick: if there is only one task running, set
> it the TF flag.
>
> But anyway, that's an optimisation. We can start with setting that flag
> on every task in that cpuset.

So long as we start without the new knob.

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