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

Vladimir Davydov
2015-11-20T10:59:19Z

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@cmpxchg.org>

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

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