Re: [PATCH] memcg: event control at vmpressure.

[Kyungmin Park <kmpark@infradead.org>]

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On Tue, Jun 11, 2013 at 9:17 AM, Anton Vorontsov <anton@enomsg.org> wrote:

> On Mon, Jun 10, 2013 at 05:12:58PM +0200, Michal Hocko wrote:
> > > +           if (level >= ev->level && level != vmpr->current_level) {
> > >                     eventfd_signal(ev->efd, 1);
> > >                     signalled = true;
> > > +                   vmpr->current_level = level;
> >
> > This would mean that you send a signal for, say, VMPRESSURE_LOW, then
> > the reclaim finishes and two days later when you hit the reclaim again
> > you would simply miss the event, right?
> >
> > So, unless I am missing something, then this is plain wrong.
>
> Yup, in it current version, it is not acceptable. For example, sometimes
> we do want to see all the _LOW events, since _LOW level shows not just the
> level itself, but the activity (i.e. reclaiming process).
>
> There are a few ways to make both parties happy, though.
>
> If the app wants to implement the time-based throttling, then just close
> the fd and sleep for needed amount of time (or do not read from the
> eventfd -- kernel then will just increment the eventfd counter, so there
> won't be context switches at the least). Doing the time-based throttling
> in the kernel won't buy us much, I believe.
>
> Or, if you still want the "one-shot"/"edge-triggered" events (which might
> make perfect sense for medium and critical levels), then I'd propose to
> add some additional flag when you register the event, so that the old
> behaviour would be still available for those who need it. This approach I
> think is the best one.
>
> Ok we will prepare this way and resend it.

Thank you,
Kyungmin Park

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