Re: [PATCH v2] vmpressure: implement strict mode

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 28 Jun 2013 11:55:47 -0700
Anton Vorontsov <anton@enomsg.org> wrote:

> On Fri, Jun 28, 2013 at 02:45:07PM -0400, Luiz Capitulino wrote:
> > On Fri, 28 Jun 2013 10:09:17 -0700
> > Anton Vorontsov <anton@enomsg.org> wrote:
> > 
> > > So, I would now argue that the current scheme is perfectly OK and can do
> > > everything you can do with the "strict" one,
> > 
> > I forgot commenting this bit. This is not true, because I don't want a
> > low fd to be notified on critical level. The current interface just
> > can't do that.
> 
> Why can't you use poll() and demultiplex the events? Check if there is an
> event in the crit fd, and if there is, then just ignore all the rest.

This may be a valid workaround for current kernels, but application
behavior will be different among kernels with a different number of
events.

Say, we events on top of critical. Then crit fd will now be
notified for cases where it didn't use to on older kernels.

> > However, it *is* possible to make non-strict work on strict if we make
> > strict default _and_ make reads on memory.pressure_level return
> > available events. Just do this on app initialization:
> > 
> > for each event in memory.pressure_level; do
> > 	/* register eventfd to be notified on "event" */
> > done
> 
> This scheme registers "all" events.

Yes, because I thought that's the user-case that matters for activity
manager :)

> Here is more complicated case:
> 
> Old kernels, pressure_level reads:
> 
>   low, med, crit
> 
> The app just wants to listen for med level.
> 
> New kernels, pressure_level reads:
> 
>   low, FOO, med, BAR, crit
> 
> How would application decide which of FOO and BAR are ex-med levels?

What you meant by ex-med?

Let's not over-design. I agree that allowing the API to be extended
is a good thing, but we shouldn't give complex meaning to events,
otherwise we're better with a numeric scale instead.

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