linux-kernel - Re: [PATCH] tcp: fix TCP socks unreleased in BBR mode

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Date:   Wed, 3 Jun 2020 13:05:05 +0800
From:   Jason Xing <kerneljasonxing@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, liweishi@...ishou.com,
        Shujin Li <lishujin@...ishou.com>
Subject: Re: [PATCH] tcp: fix TCP socks unreleased in BBR mode

Hi Eric,

I'm still trying to understand what you're saying before. Would this
be better as following:
1) discard the tcp_internal_pacing() function.
2) remove where the tcp_internal_pacing() is called in the
__tcp_transmit_skb() function.

If we do so, we could avoid 'too late to give up pacing'. Meanwhile,
should we introduce the tcp_wstamp_ns socket field as commit
(864e5c090749) does?

Thanks,
Jason

On Wed, Jun 3, 2020 at 10:44 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Jun 2, 2020 at 7:42 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > I agree with you. The upstream has already dropped and optimized this
> > part (commit 864e5c090749), so it would not happen like that. However
> > the old kernels like LTS still have the problem which causes
> > large-scale crashes on our thousands of machines after running for a
> > long while. I will send the fix to the correct tree soon :)
>
> If you run BBR at scale (thousands of machines), you probably should
> use sch_fq instead of internal pacing,
> just saying ;)
>
>
> >
> > Thanks again,
> > Jason
> >
> > On Wed, Jun 3, 2020 at 10:29 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Tue, Jun 2, 2020 at 6:53 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> > > >
> > > > Hi Eric,
> > > >
> > > > I'm sorry that I didn't write enough clearly. We're running the
> > > > pristine 4.19.125 linux kernel (the latest LTS version) and have been
> > > > haunted by such an issue. This patch is high-important, I think. So
> > > > I'm going to resend this email with the [patch 4.19] on the headline
> > > > and cc Greg.
> > >
> > > Yes, please always give for which tree a patch is meant for.
> > >
> > > Problem is that your patch is not correct.
> > > In these old kernels, tcp_internal_pacing() is called _after_ the
> > > packet has been sent.
> > > It is too late to 'give up pacing'
> > >
> > > The packet should not have been sent if the pacing timer is queued
> > > (otherwise this means we do not respect pacing)
> > >
> > > So the bug should be caught earlier. check where tcp_pacing_check()
> > > calls are missing.
> > >
> > >
> > >
> > > >
> > > >
> > > > Thanks,
> > > > Jason
> > > >
> > > > On Tue, Jun 2, 2020 at 9:05 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > > >
> > > > > On Tue, Jun 2, 2020 at 1:05 AM <kerneljasonxing@...il.com> wrote:
> > > > > >
> > > > > > From: Jason Xing <kerneljasonxing@...il.com>
> > > > > >
> > > > > > TCP socks cannot be released because of the sock_hold() increasing the
> > > > > > sk_refcnt in the manner of tcp_internal_pacing() when RTO happens.
> > > > > > Therefore, this situation could increase the slab memory and then trigger
> > > > > > the OOM if the machine has beening running for a long time. This issue,
> > > > > > however, can happen on some machine only running a few days.
> > > > > >
> > > > > > We add one exception case to avoid unneeded use of sock_hold if the
> > > > > > pacing_timer is enqueued.
> > > > > >
> > > > > > Reproduce procedure:
> > > > > > 0) cat /proc/slabinfo | grep TCP
> > > > > > 1) switch net.ipv4.tcp_congestion_control to bbr
> > > > > > 2) using wrk tool something like that to send packages
> > > > > > 3) using tc to increase the delay in the dev to simulate the busy case.
> > > > > > 4) cat /proc/slabinfo | grep TCP
> > > > > > 5) kill the wrk command and observe the number of objects and slabs in TCP.
> > > > > > 6) at last, you could notice that the number would not decrease.
> > > > > >
> > > > > > Signed-off-by: Jason Xing <kerneljasonxing@...il.com>
> > > > > > Signed-off-by: liweishi <liweishi@...ishou.com>
> > > > > > Signed-off-by: Shujin Li <lishujin@...ishou.com>
> > > > > > ---
> > > > > >  net/ipv4/tcp_output.c | 3 ++-
> > > > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > > > > > index cc4ba42..5cf63d9 100644
> > > > > > --- a/net/ipv4/tcp_output.c
> > > > > > +++ b/net/ipv4/tcp_output.c
> > > > > > @@ -969,7 +969,8 @@ static void tcp_internal_pacing(struct sock *sk, const struct sk_buff *skb)
> > > > > >         u64 len_ns;
> > > > > >         u32 rate;
> > > > > >
> > > > > > -       if (!tcp_needs_internal_pacing(sk))
> > > > > > +       if (!tcp_needs_internal_pacing(sk) ||
> > > > > > +           hrtimer_is_queued(&tcp_sk(sk)->pacing_timer))
> > > > > >                 return;
> > > > > >         rate = sk->sk_pacing_rate;
> > > > > >         if (!rate || rate == ~0U)
> > > > > > --
> > > > > > 1.8.3.1
> > > > > >
> > > > >
> > > > > Hi Jason.
> > > > >
> > > > > Please do not send patches that do not apply to current upstream trees.
> > > > >
> > > > > Instead, backport to your kernels the needed fixes.
> > > > >
> > > > > I suspect that you are not using a pristine linux kernel, but some
> > > > > heavily modified one and something went wrong in your backports.
> > > > > Do not ask us to spend time finding what went wrong.
> > > > >
> > > > > Thank you.