netdev - Re: BBR and TCP internal pacing causing interrupt storm with pfifo

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Date:   Tue, 9 Oct 2018 10:00:49 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Gasper Zejn <zelo.zejn@...il.com>, Kevin Yang <yyd@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: Re: BBR and TCP internal pacing causing interrupt storm with
 pfifo_fast



On 10/09/2018 09:38 AM, Gasper Zejn wrote:
> Hello,
> 
> I am seeing interrupt storms of over 100k-900k local timer interrupts
> when changing between network devices or networks with open TCP
> connections when not using sch_fq (I was using pfifo_fast). Using sch_fq
> makes the bug with interrupt storm go away.
> 

That is for what kind of traffic ?

If your TCP flows send 100k-3M packets per second, then yes, the pacing timers
could be setup in the 100k-900k range.

> The interrupts all called tcp_pace_kick (according to perf), which seems
> to return HRTIMER_NORESTART, but apparently somewhere calls another
> function, that does restart the timer.
> 
> The bug is fairly easy to reproduce. Congestion control needs to be BBR,
> network scheduler was pfifo_fast, and there need to be open TCP
> connections when changing network in such a way that TCP connections
> cannot continue to work (eg. different client IP addresses). The more
> connections the more interrupts. The connection handling code will cause
> interrupt storm, which eventually sets down as the connections time out.
> It is a bit annoying as high interrupt rate does not show as load. I
> successfully reproduced this with 4.18.12, but this has been happening
> for some time, with previous versions of kernel too.
> 
> 
> I'd like to thank you for the comment regarding use of sch_fq with BBR
> above the tcp_needs_internal_pacing function. It has pointed me in the
> direction to find the workaround.
>

Well, BBR has been very clear about sch_fq being the best packet scheduler

net/ipv4/tcp_bbr.c currently says :

/* ...
 *
 * NOTE: BBR might be used with the fq qdisc ("man tc-fq") with pacing enabled,
 * otherwise TCP stack falls back to an internal pacing using one high
 * resolution timer per TCP socket and may use more resources.
 */