netdev - Re: [PATCH 08/14] net: tcp_memcontrol: sanitize tcp memory accounting callbacks

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Date:	Fri, 20 Nov 2015 13:58:57 +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 08/14] net: tcp_memcontrol: sanitize tcp memory
 accounting callbacks

On Thu, Nov 12, 2015 at 06:41:27PM -0500, Johannes Weiner wrote:
> There won't be a tcp control soft limit, so integrating the memcg code
> into the global skmem limiting scheme complicates things
> unnecessarily. Replace this with simple and clear charge and uncharge
> calls--hidden behind a jump label--to account skb memory.
> 
> Note that this is not purely aesthetic: as a result of shoehorning the
> per-memcg code into the same memory accounting functions that handle
> the global level, the old code would compare the per-memcg consumption
> against the smaller of the per-memcg limit and the global limit. This
> allowed the total consumption of multiple sockets to exceed the global
> limit, as long as the individual sockets stayed within bounds. After
> this change, the code will always compare the per-memcg consumption to
> the per-memcg limit, and the global consumption to the global limit,
> and thus close this loophole.
> 
> Without a soft limit, the per-memcg memory pressure state in sockets
> is generally questionable. However, we did it until now, so we
> continue to enter it when the hard limit is hit, and packets are
> dropped, to let other sockets in the cgroup know that they shouldn't
> grow their transmit windows, either. However, keep it simple in the
> new callback model and leave memory pressure lazily when the next
> packet is accepted (as opposed to doing it synchroneously when packets
> are processed). When packets are dropped, network performance will
> already be in the toilet, so that should be a reasonable trade-off.
> 
> As described above, consumption is now checked on the per-memcg level
> and the global level separately. Likewise, memory pressure states are
> maintained on both the per-memcg level and the global level, and a
> socket is considered under pressure when either level asserts as much.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

It leaves the legacy functionality intact, while making the code look
much better.

Reviewed-by: Vladimir Davydov <vdavydov@...tuozzo.com>
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