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

From: Neal Cardwell <ncardwell@google.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Jason Xing <kerneljasonxing@gmail.com>,
	David Miller <davem@davemloft.net>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	netdev <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	liweishi@kuaishou.com, Shujin Li <lishujin@kuaishou.com>
Subject: Re: [PATCH] tcp: fix TCP socks unreleased in BBR mode
Date: Wed, 3 Jun 2020 08:02:13 -0400	[thread overview]
Message-ID: <CADVnQy=RJfmzHR15DyWdydFAqSqVmFhaW4_cgYYAgnixEa5DNQ@mail.gmail.com> (raw)
In-Reply-To: <CANn89iJfLM2Hz69d9qOZoRKwzzCCpgVRZ1zbTTbg4vGvSAEZ-w@mail.gmail.com>

On Wed, Jun 3, 2020 at 1:44 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Tue, Jun 2, 2020 at 10:05 PM Jason Xing <kerneljasonxing@gmail.com> 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.

Perhaps this could also happen from some of the retransmission code
paths that don't use tcp_xmit_retransmit_queue()? Perhaps
tcp_retransmit_timer() (RTO) and  tcp_send_loss_probe() TLP? It seems
they could indirectly cause a call to __tcp_transmit_skb() and thus
tcp_internal_pacing() without first checking if the pacing timer was
already armed?

neal