Messages in this thread | | | Date | Fri, 24 Jun 2022 21:16:05 +0800 | Subject | Re: Perf regression from scheduler load_balance rework in 5.5? | From | Zhang Qiao <> |
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Hi, 在 2022/6/24 16:22, Vincent Guittot 写道: > On Thu, 23 Jun 2022 at 21:50, David Chen <david.chen@nutanix.com> wrote: >> >> Hi, >> >> I'm working on upgrading our kernel from 4.14 to 5.10 >> However, I'm seeing performance regression when doing rand read from windows client through smbd >> with a well cached file. >> >> One thing I noticed is that on the new kernel, the smbd thread doing socket I/O tends to stay on >> the same cpu core as the net_rx softirq, where as in the old kernel it tends to be moved around >> more randomly. And when they are on the same cpu, it tends to saturate the cpu more and causes >> performance to drop. >> >> For example, here's the duration (ns) the thread spend on each cpu I captured using bpftrace >> On 4.14: >> @cputime[7]: 20741458382 >> @cputime[0]: 25219285005 >> @cputime[6]: 30892418441 >> @cputime[5]: 31032404613 >> @cputime[3]: 33511324691 >> @cputime[1]: 35564174562 >> @cputime[4]: 39313421965 >> @cputime[2]: 55779811909 (net_rx cpu) >> >> On 5.10: >> @cputime[3]: 2150554823 >> @cputime[5]: 3294276626 >> @cputime[7]: 4277890448 >> @cputime[4]: 5094586003 >> @cputime[1]: 6058168291 >> @cputime[0]: 14688093441 >> @cputime[6]: 17578229533 >> @cputime[2]: 223473400411 (net_rx cpu) >> >> I also tried setting the cpu affinity of the smbd thread away from the net_rx cpu and indeed that >> seems to bring the perf on par with old kernel.
I observed the same problem for the past two weeks.
>> >> I noticed that there's scheduler load_balance rework in 5.5, so I did the test on 5.4 and 5.5 and >> it did show the behavior changed between 5.4 and 5.5. > > Have you tested v5.18 ? several improvements happened since v5.5 > >> >> Anyone know how to work around this? > > Have you enabled IRQ_TIME_ACCOUNTING ?
CONFIG_IRQ_TIME_ACCOUNTING=y.
> > When the time spent under interrupt becomes significant, scheduler > migrate task on another cpu
My board has two cpus, and i used iperf3 to test upload bandwidth,then I saw the same situation, the iperf3 thread run on the same cpu as the NET_RX softirq.
After debug in find_busiest_group(), i noticed when the cpu(env->idle is CPU_IDLE or CPU_NEWLY_IDLE) try to pull task, the busiest->group_type == group_fully_busy, busiest->sum_h_nr_running == 1, local->group_type==group_has_spare, and the loadbalance will failed at find_busiest_group(), as follows:
find_busiest_group(): ... if (busiest->group_type != group_overloaded) { .... if (busiest->sum_h_nr_running == 1) goto out_balanced; ----> loadbalance will returned at here. ....
Thanks, Qiao
> Vincent>> >> Thanks, >> David > . >
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