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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Tue, 11 Jun 2013 08:21:24 +0200
Michal Hocko <mhocko@suse.cz> wrote:

> On Mon 10-06-13 17:17:47, Anton Vorontsov 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).
> 
> That makes sense to me.
> 
> > Doing the time-based throttling in the kernel won't buy us much, I
> > believe.
> 
> Yes.
>  
> > 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.
> 
> Hmm, how would one-shot even differ from a single open, register, read
> and close?

Agreed.

A different solution would be to have a simple state machine and notify
user-space on state transitions.

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