linux-kernel - Re: [PATCH v2] vmpressure: implement strict mode

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Date:	Fri, 28 Jun 2013 11:55:47 -0700
From:	Anton Vorontsov <anton@...msg.org>
To:	Luiz Capitulino <lcapitulino@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, mhocko@...e.cz, kmpark@...radead.org,
	hyunhee.kim@...sung.com
Subject: Re: [PATCH v2] vmpressure: implement strict mode

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@...msg.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.

> 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. 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?

Anton
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