Hi Namhyung,
>> I re-written the patch as following. hackbench/aim9 doest show clean
>> performance change.
>> Actually we can get some profit. it also will be very slight. :)
>> BTW, it still need another patch before apply this. Just to show the logical.
>>
>> ===========
>>> From 145ff27744c8ac04eda056739fe5aa907a00877e Mon Sep 17 00:00:00 2001
>> From: Alex Shi <alex....@intel.com>
>> Date: Fri, 11 Jan 2013 16:49:03 +0800
>> Subject: [PATCH 3/7] sched: select_idle_sibling optimization
>>
>> Current logical in this function will insist to wake up the task in a
>> totally idle group, otherwise it would rather back to previous cpu.
>
> Or current cpu depending on result of wake_affine(), right?
>
>>
>> The new logical will try to wake up the task on any idle cpu in the same
>> cpu socket (in same sd_llc), while idle cpu in the smaller domain has
>> higher priority.
>
> But what about SMT domain?
The previous approach also descended till the SMT domain.Here we start
from the SMT domain.
You could check with /proc/schedstat as to which are the different
domains the cpu is a part of and SMT domain happens to be domain0.As far
as i know for_each_lower_domain will descend till domain0.
>
> I mean it seems that the code prefers running a task on a idle cpu which
> is a sibling thread in the same core rather than running it on an idle
> cpu in another idle core. I guess we didn't do that before.
>
>>
>> It should has some help on burst wake up benchmarks like aim7.
>>
>> Original-patch-by: Preeti U Murthy <pre...@linux.vnet.ibm.com>
>> Signed-off-by: Alex Shi <alex....@intel.com>
>> ---
>> kernel/sched/fair.c | 40 +++++++++++++++++++---------------------
>> 1 files changed, 19 insertions(+), 21 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index e116215..fa40e49 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3253,13 +3253,13 @@ find_idlest_cpu(struct sched_group *group, struct
>> task_struct *p, int this_cpu)
>> /*
>> * Try and locate an idle CPU in the sched_domain.
>> */
>> -static int select_idle_sibling(struct task_struct *p)
>> +static int select_idle_sibling(struct task_struct *p,
>> + struct sched_domain *affine_sd, int sync)
>
> Where are these arguments used?
>
>
>> {
>> int cpu = smp_processor_id();
>> int prev_cpu = task_cpu(p);
>> struct sched_domain *sd;
>> struct sched_group *sg;
>> - int i;
>>
>> /*
>> * If the task is going to be woken-up on this cpu and if it is
>> @@ -3281,27 +3281,25 @@ static int select_idle_sibling(struct task_struct *p)
>> /*
>> * Otherwise, iterate the domains and find an elegible idle cpu.
>> */
>> - sd = rcu_dereference(per_cpu(sd_llc, prev_cpu));
>> - for_each_lower_domain(sd) {
>> + for_each_domain(prev_cpu, sd) {
>
> Always start from the prev_cpu?
>
>
>> sg = sd->groups;
>> do {
>> - if (!cpumask_intersects(sched_group_cpus(sg),
>> - tsk_cpus_allowed(p)))
>> - goto next;
>> -
>> - for_each_cpu(i, sched_group_cpus(sg)) {
>> - if (!idle_cpu(i))
>> - goto next;
>> - }
>> -
>> - prev_cpu = cpumask_first_and(sched_group_cpus(sg),
>> - tsk_cpus_allowed(p));
>> - goto done;
>> -next:
>> - sg = sg->next;
>> - } while (sg != sd->groups);
>> + int nr_busy = atomic_read(&sg->sgp->nr_busy_cpus);
>> + int i;
>> +
>> + /* no idle cpu in the group */
>> + if (nr_busy == sg->group_weight)
>> + continue;
>
> Maybe we can skip local group since it's a bottom-up search so we know
> there's no idle cpu in the lower domain from the prior iteration.
We could have done this for the current logic because it checks for an
*idle* group.The local group would definitely fail this test.But here we
need to check the local group also because we are looking for an idle cpu.
Regards
Preeti U Murthy
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