linux-kernel - Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

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Message-ID: <20121121084603.GA18159@lizard>
Date:	Wed, 21 Nov 2012 00:46:03 -0800
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Pekka Enberg <penberg@...nel.org>,
	Mel Gorman <mgorman@...e.de>,
	Leonid Moiseichuk <leonid.moiseichuk@...ia.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	John Stultz <john.stultz@...aro.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	patches@...aro.org, kernel-team@...roid.com,
	linux-man@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

On Wed, Nov 21, 2012 at 12:27:28PM +0400, Glauber Costa wrote:
> On 11/20/2012 10:23 PM, David Rientjes wrote:
> > Anton can correct me if I'm wrong, but I certainly don't think this is 
> > where mempressure is headed: I don't think any accounting needs to be done

Yup, I'd rather not do any accounting, at least not in bytes.

> > and, if it is, it's a design issue that should be addressed now rather 
> > than later.  I believe notifications should occur on current's mempressure 
> > cgroup depending on its level of reclaim: nobody cares if your memcg has a 
> > limit of 64GB when you only have 32GB of RAM, we'll want the notification.
> 
> My main concern is that to trigger those notifications, one would have
> to first determine whether or not the particular group of tasks is under
> pressure.

As far as I understand, the notifications will be triggered by a process
that tries to allocate memory. So, effectively that would be a per-process
pressure.

So, if one process in a group is suffering, we notify that "a process in a
group is under pressure", and the notification goes to a cgroup listener

> And to do that, we need to somehow know how much memory we are
> using, and how much we are reclaiming, etc. On a system-wide level, we
> have this information. On a grouplevel, this is already accounted by memcg.
> 
> In fact, the current code already seems to rely on memcg:
> 
> +	vmpressure(sc->target_mem_cgroup,
> +		   sc->nr_scanned - nr_scanned, nr_reclaimed);

Well, I'm yet unsure about the details, but I guess in "mempressure"
cgroup approach, this will be derived from the current->, i.e. a task.

But note that we won't report pressure to a memcg cgroup, we will notify
only mempressure cgroup. But a process can be in both of them
simultaneously. In the code, the mempressure and memcg will not depend on
each other.

> Now, let's start simple: Assume we will have a different cgroup.
> We want per-group pressure notifications for that group. How would you
> determine that the specific group is under pressure?

If a process that tries to allocate memory & causes reclaim is a part of
the cgroup, then cgroup has a pressure.

At least that's very brief understanding of the idea, details to be
investigated... But I welcome David to comment whether I got everything
correctly. :)

Thanks,
Anton.
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