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

[Eric Dumazet]

On Tue, Jun 2, 2020 at 10:05 PM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>
> 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?
>

Please do not top-post on netdev mailing list.


I basically suggested double-checking which point in TCP could end up
calling tcp_internal_pacing()
while the timer was already armed.

I guess this is mtu probing.

Please try the following patch : If we still have another bug, a
WARNING should give us a stack trace.


diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index cc4ba42052c21b206850594db6751810d8fc72b4..8f4081b228486305222767d4d118b9b6ed0ffda3
100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -977,12 +977,26 @@ static void tcp_internal_pacing(struct sock *sk,
const struct sk_buff *skb)

        len_ns = (u64)skb->len * NSEC_PER_SEC;
        do_div(len_ns, rate);
+
+       /* If hrtimer is already armed, then our caller has not properly
+        * used tcp_pacing_check().
+        */
+       if (unlikely(hrtimer_is_queued(&tcp_sk(sk)->pacing_timer))) {
+               WARN_ON_ONCE(1);
+               return;
+       }
        hrtimer_start(&tcp_sk(sk)->pacing_timer,
                      ktime_add_ns(ktime_get(), len_ns),
                      HRTIMER_MODE_ABS_PINNED_SOFT);
        sock_hold(sk);
 }

+static bool tcp_pacing_check(const struct sock *sk)
+{
+       return tcp_needs_internal_pacing(sk) &&
+              hrtimer_is_queued(&tcp_sk(sk)->pacing_timer);
+}
+
 static void tcp_update_skb_after_send(struct tcp_sock *tp, struct sk_buff *skb)
 {
        skb->skb_mstamp = tp->tcp_mstamp;
@@ -2117,6 +2131,9 @@ static int tcp_mtu_probe(struct sock *sk)
        if (!tcp_can_coalesce_send_queue_head(sk, probe_size))
                return -1;

+       if (tcp_pacing_check(sk))
+               return -1;
+
        /* We're allowed to probe.  Build it now. */
        nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC, false);
        if (!nskb)
@@ -2190,11 +2207,6 @@ static int tcp_mtu_probe(struct sock *sk)
        return -1;
 }

-static bool tcp_pacing_check(const struct sock *sk)
-{
-       return tcp_needs_internal_pacing(sk) &&
-              hrtimer_is_queued(&tcp_sk(sk)->pacing_timer);
-}

 /* TCP Small Queues :
  * Control number of packets in qdisc/devices to two packets / or ~1 ms.



> Thanks,
> Jason
>
> On Wed, Jun 3, 2020 at 10:44 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Tue, Jun 2, 2020 at 7:42 PM Jason Xing <kerneljasonxing@xxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Jun 2, 2020 at 6:53 PM Jason Xing <kerneljasonxing@xxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Tue, Jun 2, 2020 at 1:05 AM <kerneljasonxing@xxxxxxxxx> wrote:
> > > > > > >
> > > > > > > From: Jason Xing <kerneljasonxing@xxxxxxxxx>
> > > > > > >
> > > > > > > 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@xxxxxxxxx>
> > > > > > > Signed-off-by: liweishi <liweishi@xxxxxxxxxxxx>
> > > > > > > Signed-off-by: Shujin Li <lishujin@xxxxxxxxxxxx>
> > > > > > > ---
> > > > > > >  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.