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

From: Johannes Weiner
Date: Fri Nov 20 2015 - 13:43:14 EST

On Fri, Nov 20, 2015 at 01:58:57PM +0300, Vladimir Davydov wrote:
> 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@xxxxxxxxxxx>
> It leaves the legacy functionality intact, while making the code look
> much better.
> Reviewed-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>

Thank you very much!
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