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

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Date:	Fri, 28 Jun 2013 11:45:47 -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 02:25:58PM -0400, Luiz Capitulino wrote:
> > > > That's how it's expected to work, because on strict mode you're notified
> > > > for the level you registered for. So apps registering for critical, will
> > > > still be notified on critical just like before.
> > > 
> > > Suppose you introduce a new level, and the system hits this level. Before,
> > > the app would receive at least some notification for the given memory load
> > > (i.e. one of the old levels), with the new level introduced in the kernel,
> > > the app will receive no events at all.
> 
> That's not true. If an app registered for critical it will still get
> critical notification when the system is at the critical level. Just as it
> always did. No new events will change this.
> 
> With today's semantics though, new events will change when current events
> are triggered. So each new extension will cause applications to have
> different behaviors, in different kernel versions. This looks quite
> undesirable to me.

I'll try to explain it again.

Old behaviour:

low -> event
  x <- but the system is at this unnamed level, between low and med
med
crit


We add a level:

low
low-med <- system at this state, we send an event, but the old app does
           not know about it, so it won't receive *any* notifications. (In
	   older kernels it would receive low level notification
med
crit

You really don't see a problem here?

I see the problem, and I see these solutions:

1. Use current scheme.
2. Add versioning.
3. Never change the levels (how can we know?)

Anton
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