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.
*/
