RE: Perf regression from scheduler load_balance rework in 5.5?
From: David Chen
Date: Wed Jun 29 2022 - 17:46:04 EST
> -----Original Message-----
> From: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Sent: Monday, June 27, 2022 4:00 AM
> To: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
> Cc: David Chen <david.chen@xxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Ingo Molnar <mingo@xxxxxxxxxx>
> Subject: Re: Perf regression from scheduler load_balance rework in 5.5?
>
> 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@xxxxxxxxxxx> 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.
>
> 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
> > > .
> > >
Hi,
I applied the patch on top of 5.10 and also enabled CONFIG_IRQ_TIME_ACCOUNTING.
And it did fix the issue I had.
Thanks,
David