Re: Perf regression from scheduler load_balance rework in 5.5?

[Zhang Qiao]

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@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. 


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
>>> .
>>>
> .
>