Messages in this thread | | | Date | Thu, 30 Jun 2022 15:02:12 +0800 | Subject | Re: Perf regression from scheduler load_balance rework in 5.5? | From | Zhang Qiao <> |
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在 2022/6/27 18:59, Vincent Guittot 写道: > Hi, > > Le vendredi 24 juin 2022 à 21:16:05 (+0800), Zhang Qiao a écrit : >> >> 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. > > Yes, you're right, we filter such case. Could you try the patch below ? > I use the misfit task state to detect cpu with reduced capacity and migrate_load > to check if it worth migration the task on the dst cpu.
Hi,
I tested with this patch, it is ok.
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 6775a117f3c1..013dcd97472b 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8757,11 +8757,19 @@ static inline void update_sg_lb_stats(struct lb_env *env, > if (local_group) > continue; > > - /* Check for a misfit task on the cpu */ > - if (env->sd->flags & SD_ASYM_CPUCAPACITY && > - sgs->group_misfit_task_load < rq->misfit_task_load) { > - sgs->group_misfit_task_load = rq->misfit_task_load; > - *sg_status |= SG_OVERLOAD; > + if (env->sd->flags & SD_ASYM_CPUCAPACITY) { > + /* Check for a misfit task on the cpu */ > + if (sgs->group_misfit_task_load < rq->misfit_task_load) { > + sgs->group_misfit_task_load = rq->misfit_task_load; > + *sg_status |= SG_OVERLOAD; > + } > + } else if ((env->idle != CPU_NOT_IDLE) && > + (group->group_weight == 1) && > + (rq->cfs.h_nr_running == 1) && > + check_cpu_capacity(rq, env->sd) && > + (sgs->group_misfit_task_load < cpu_load(rq))) { > + /* Check for a task running on a CPU with reduced capacity */ > + sgs->group_misfit_task_load = cpu_load(rq); > } > } > > @@ -8814,7 +8822,8 @@ static bool update_sd_pick_busiest(struct lb_env *env, > * CPUs in the group should either be possible to resolve > * internally or be covered by avg_load imbalance (eventually). > */ > - if (sgs->group_type == group_misfit_task && > + if ((env->sd->flags & SD_ASYM_CPUCAPACITY) && > + (sgs->group_type == group_misfit_task) && > (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) || > sds->local_stat.group_type != group_has_spare)) > return false; > @@ -9360,9 +9369,15 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s > busiest = &sds->busiest_stat; > > if (busiest->group_type == group_misfit_task) { > - /* Set imbalance to allow misfit tasks to be balanced. */ > - env->migration_type = migrate_misfit; > - env->imbalance = 1; > + if (env->sd->flags & SD_ASYM_CPUCAPACITY) { > + /* Set imbalance to allow misfit tasks to be balanced. */ > + env->migration_type = migrate_misfit; > + env->imbalance = 1; > + } else { > + /* Set group overloaded as one cpu has reduced capacity */ > + env->migration_type = migrate_load; > + env->imbalance = busiest->group_misfit_task_load; > + } > return; > } > > >> .... >> >> >> Thanks, >> Qiao >> >> >>> Vincent>> >>>> Thanks, >>>> David >>> . >>> > . >
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