netdev - Re: [PATCH v2] net: memcg: late association of sock to memcg

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Date:   Wed, 4 Mar 2020 18:18:48 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev <netdev@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] net: memcg: late association of sock to memcg

On Wed, Mar 4, 2020 at 5:36 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Mar 4, 2020 at 3:39 PM Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > If a TCP socket is allocated in IRQ context or cloned from unassociated
> > (i.e. not associated to a memcg) in IRQ context then it will remain
> > unassociated for its whole life. Almost half of the TCPs created on the
> > system are created in IRQ context, so, memory used by such sockets will
> > not be accounted by the memcg.
> >
> > This issue is more widespread in cgroup v1 where network memory
> > accounting is opt-in but it can happen in cgroup v2 if the source socket
> > for the cloning was created in root memcg.
> >
> > To fix the issue, just do the late association of the unassociated
> > sockets at accept() time in the process context and then force charge
> > the memory buffer already reserved by the socket.
> >
> > Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> > ---
> > Changes since v1:
> > - added sk->sk_rmem_alloc to initial charging.
> > - added synchronization to get memory usage and set sk_memcg race-free.
> >
> >  net/ipv4/inet_connection_sock.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >
> > diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> > index a4db79b1b643..7bcd657cd45e 100644
> > --- a/net/ipv4/inet_connection_sock.c
> > +++ b/net/ipv4/inet_connection_sock.c
> > @@ -482,6 +482,25 @@ struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern)
> >                 }
> >                 spin_unlock_bh(&queue->fastopenq.lock);
> >         }
> > +
> > +       if (mem_cgroup_sockets_enabled && !newsk->sk_memcg) {
> > +               int amt;
> > +
> > +               /* atomically get the memory usage and set sk->sk_memcg. */
> > +               lock_sock(newsk);
> > +
> > +               /* The sk has not been accepted yet, no need to look at
> > +                * sk->sk_wmem_queued.
> > +                */
> > +               amt = sk_mem_pages(newsk->sk_forward_alloc +
> > +                                  atomic_read(&sk->sk_rmem_alloc));
> > +               mem_cgroup_sk_alloc(newsk);
> > +
> > +               release_sock(newsk);
> > +
> > +               if (newsk->sk_memcg)
>
> Most sockets in accept queue should have amt == 0, so maybe avoid
> calling this thing only when amt == 0 ?
>

Thanks, will do in the next version. BTW I have tested with adding
mdelay() here and running iperf3 and I did see non-zero amt.

> Also  I would release_sock(newsk) after this, otherwise incoming
> packets could mess with newsk->sk_forward_alloc
>

I think that is fine. Once sk->sk_memcg is set then
mem_cgroup_charge_skmem() will be called for new incoming packets.
Here we just need to call mem_cgroup_charge_skmem() with amt before
sk->sk_memcg was set.

> if (amt && newsk->sk_memcg)
>       mem_cgroup_charge_skmem(newsk->sk_memcg, amt);
> release_sock(newsk);
>
> Also, I wonder if     mem_cgroup_charge_skmem() has been used at all
> these last four years
> on arches with PAGE_SIZE != 4096
>
> ( SK_MEM_QUANTUM is not anymore PAGE_SIZE, but 4096)
>

Oh so sk_mem_pages() does not really give the number of pages. Yeah
this needs a fix for non-4906 page size architectures. Though I can
understand why this has not been caught yet. Network memory accounting
is opt-in in cgroup v1 and most of the users still use v1. In cgroup
v2, it is enabled and there is no way to opt-out. Facebook is a
well-known v2 user and it seems like they don't have non-4096 page
size arch systems.