Re: [PATCH] sched: add heuristic logic to pick idle peers

[Lei Wen]

Hi Michael,

On Mon, Jun 17, 2013 at 2:44 PM, Michael Wang
<wangyun@xxxxxxxxxxxxxxxxxx> wrote:
> On 06/17/2013 01:08 PM, Lei Wen wrote:
>> Hi Michael,
>>
>> On Mon, Jun 17, 2013 at 11:27 AM, Michael Wang
>> <wangyun@xxxxxxxxxxxxxxxxxx> wrote:
>>> Hi, Lei
>>>
>>> On 06/17/2013 10:21 AM, Lei Wen wrote:
>>>> nr_busy_cpus in sched_group_power structure cannot present the purpose
>>>> for judging below statement:
>>>> "this cpu's scheduler group has multiple busy cpu's exceeding
>>>>  the group's power."
>>>>
>>>> But only could tell how many cpus is doing their jobs for currently.
>>>
>>> AFAIK, this nr_busy_cpus presents how many cpus in local group are not
>>> idle, the logical here in nohz_kick_needed() is:
>>>
>>>         if domain cpus share resources and at least 2 cpus in
>>>         local group are not idle, prefer to do balance.
>>>
>>
>> Seems reasonable for me. But this comment is conflicted with current documented
>> one. Do we need to modify the comment anyway, as previous says nr_busy>1 is
>> "scheduler group has multiple busy cpu;s exceeding the group's power"?
>
> I agree it doesn't make sense (to me), the logical here only make sure
> there are at least 2 non-idle cpus in local group, we may need some more
> powerful folks to confirm that point.
>
>>
>>> And the idea behind is, we catch the timing when there are idle-cpu and
>>> busy-group and task-moving may cost low.
>>
>> Since there is only one task over runqueue now, then why we could need the
>> load balance any way?...
>
> IMHO, this is just shot in the darkness... like 'I think in such cases
> the chances of requiring a balance will be high', but the problem is,
> the logical is already in mainline for some reasons, if we want to say
> that is wrong, then we need to collect enough proof...

I see...

>
>>
>>>
>>> Your change will remove this timing for balance, I think you may need
>>> some test to prove that this patch will make things better.
>>
>> I see. Yes, test data is always good. :)
>> Do you have any suggestion like using what kind of test program to
>> collect this data?
>
> Any workload which require a good balance to check whether the patch
> cause damage, any workload which is latency-sensitive to check whether
> the patch bring benefit, what about kernbench with enough threads firstly?
>
> Actually all the popular benchmark worth a try, until some improvement
> was found, if after all the test, still no benefit located, then the
> idea may have to be dropped...

Thanks for suggestion, I would do some test locally first and try to
get such proof. :)

Thanks,
Lei


>
> Regards,
> Michael Wang
>
>>
>> Thanks,
>> Lei
>>
>>>
>>> Regards,
>>> Michael Wang
>>>
>>>>
>>>> However, the original purpose to add this logic still looks good.
>>>> So we move this kind of logic to find_new_ilb, so that we could pick
>>>> out peer from our sharing resource domain whenever possible.
>>>>
>>>> Signed-off-by: Lei Wen <leiwen@xxxxxxxxxxx>
>>>> ---
>>>>  kernel/sched/fair.c |   28 ++++++++++++++++++++++------
>>>>  1 file changed, 22 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index c61a614..64f9120 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -5368,10 +5368,31 @@ static struct {
>>>>       unsigned long next_balance;     /* in jiffy units */
>>>>  } nohz ____cacheline_aligned;
>>>>
>>>> +/*
>>>> + * Add the heuristic logic to try waking up idle cpu from
>>>> + * those peers who share resources with us, so that the
>>>> + * cost would be brought to minimum.
>>>> + */
>>>>  static inline int find_new_ilb(int call_cpu)
>>>>  {
>>>> -     int ilb = cpumask_first(nohz.idle_cpus_mask);
>>>> +     int ilb = nr_cpu_ids;
>>>> +     struct sched_domain *sd;
>>>> +
>>>> +     rcu_read_lock();
>>>> +     for_each_domain(call_cpu, sd) {
>>>> +             /* We loop till sched_domain no longer share resource */
>>>> +             if (!(sd->flags & SD_SHARE_PKG_RESOURCES)) {
>>>> +                     ilb = cpumask_first(nohz.idle_cpus_mask);
>>>> +                     break;
>>>> +             }
>>>>
>>>> +             /* else, we would try to pick the idle cpu from peers first */
>>>> +             ilb = cpumask_first_and(nohz.idle_cpus_mask,
>>>> +                             sched_domain_span(sd));
>>>> +             if (ilb < nr_cpu_ids)
>>>> +                     break;
>>>> +     }
>>>> +     rcu_read_unlock();
>>>>       if (ilb < nr_cpu_ids && idle_cpu(ilb))
>>>>               return ilb;
>>>>
>>>> @@ -5620,8 +5641,6 @@ end:
>>>>   * Current heuristic for kicking the idle load balancer in the presence
>>>>   * of an idle cpu is the system.
>>>>   *   - This rq has more than one task.
>>>> - *   - At any scheduler domain level, this cpu's scheduler group has multiple
>>>> - *     busy cpu's exceeding the group's power.
>>>>   *   - For SD_ASYM_PACKING, if the lower numbered cpu's in the scheduler
>>>>   *     domain span are idle.
>>>>   */
>>>> @@ -5659,9 +5678,6 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
>>>>               struct sched_group_power *sgp = sg->sgp;
>>>>               int nr_busy = atomic_read(&sgp->nr_busy_cpus);
>>>>
>>>> -             if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
>>>> -                     goto need_kick_unlock;
>>>> -
>>>>               if (sd->flags & SD_ASYM_PACKING && nr_busy != sg->group_weight
>>>>                   && (cpumask_first_and(nohz.idle_cpus_mask,
>>>>                                         sched_domain_span(sd)) < cpu))
>>>>
>>>
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>
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