Re: [PATCH v10 10/11] sched: add SD_PREFER_SIBLING for SMT level

[Vincent Guittot]

On 2 March 2015 at 12:52, Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx> wrote:
> * Vincent Guittot <vincent.guittot@xxxxxxxxxx> [2015-02-27 16:54:13]:
>
>> Add the SD_PREFER_SIBLING flag for SMT level in order to ensure that
>> the scheduler will put at least 1 task per core.
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>> Reviewed-by: Preeti U. Murthy <preeti@xxxxxxxxxxxxxxxxxx>
>> ---
>>  kernel/sched/core.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 29f7037..753f0a2 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -6240,6 +6240,7 @@ sd_init(struct sched_domain_topology_level *tl, int cpu)
>>        */
>>
>>       if (sd->flags & SD_SHARE_CPUCAPACITY) {
>> +             sd->flags |= SD_PREFER_SIBLING;
>>               sd->imbalance_pct = 110;
>>               sd->smt_gain = 1178; /* ~15% */
>>
>
> Prefer siblings logic dates back to https://lkml.org/lkml/2009/8/27/210
> and only used in update_sd_lb_stats() where we have
>
> if (child && child->flags & SD_PREFER_SIBLING)
>          prefer_sibling = 1;
>
> However what confuses me is why should we even look at a child domain's
> flag to balance tasks across the current sched domain? Why cant we just
> set and use a sd flag at current level than to look at child domain
> flag?

Peter,
have you got some insight about the reason ?

egarding SMT, I see one advantage : the prefer sibling flag only need
to be set if SMT is enable and a smt sched_domain is present but it
implies some action at the parent level. If the flag was directly set
at parent level, the behavior linked to SD_PREFER_SIBLING flag will be
applied whatever the presence or not of the SMT domain.

Regards,
Vincent


>
>> --
>> 1.9.1
>>
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> --
> Thanks and Regards
> Srikar Dronamraju
>
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