netdev - Re: [PATCH 14/14] mm: memcontrol: hook up vmpressure to socket pressure

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Message-ID: <20151115135457.GM31308@esperanza>
Date:	Sun, 15 Nov 2015 16:54:57 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	<netdev@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<kernel-team@...com>
Subject: Re: [PATCH 14/14] mm: memcontrol: hook up vmpressure to socket
 pressure

On Thu, Nov 12, 2015 at 06:41:33PM -0500, Johannes Weiner wrote:
> Let the networking stack know when a memcg is under reclaim pressure
> so that it can clamp its transmit windows accordingly.
> 
> Whenever the reclaim efficiency of a cgroup's LRU lists drops low
> enough for a MEDIUM or HIGH vmpressure event to occur, assert a
> pressure state in the socket and tcp memory code that tells it to curb
> consumption growth from sockets associated with said control group.
> 
> vmpressure events are naturally edge triggered, so for hysteresis
> assert socket pressure for a second to allow for subsequent vmpressure
> events to occur before letting the socket code return to normal.

AFAICS, in contrast to v1, now you don't modify vmpressure behavior,
which means socket_pressure will only be set when cgroup hits its
high/hard limit. On tightly packed system, this is unlikely IMO -
cgroups will mostly experience pressure due to memory shortage at the
global level and/or their low limit configuration, in which case no
vmpressure events will be triggered and therefore tcp window won't be
clamped accordingly.

May be, we could use a per memcg slab shrinker to detect memory
pressure? This looks like abusing shrinkers API though.

Thanks,
Vladimir
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