On 09/25/2013 04:56 PM, Mike Galbraith wrote:
> On Wed, 2013-09-25 at 09:53 +0200, Peter Zijlstra wrote:
>> Subject: sched: Avoid select_idle_sibling() for wake_affine(.sync=true)
>> From: Peter Zijlstra <pet...@infradead.org>
>> Date: Wed Sep 25 08:28:39 CEST 2013
>>
>> When a task is the only running task and does a sync wakeup; avoid
>> going through select_idle_sibling() as it doesn't know the current CPU
>> is going to be idle shortly.
>>
>> Without this two sync wakers will ping-pong between CPUs for no
>> reason.
>
> That will make pipe-test go fugly -> pretty, and help very fast/light
> localhost network, but eat heavier localhost overlap recovery. We need
> a working (and cheap) overlap detector scheme, so we can know when there
> is enough to be worth going after.
>
> (I sent you some lmbench numbers offline a while back showing the
> two-faced little <b-word> in action, doing both good and evil)
It seems like the choice between the overhead and a little possibility
to balance the load :)
Like the case when we have:
core0 sg core1 sg
cpu0 cpu1 cpu2 cpu3
waker busy idle idle
If the sync wakeup was on cpu0, we can:
1. choose cpu in core1 sg like we did usually
some overhead but tend to make the load a little balance
core0 sg core1 sg
cpu0 cpu1 cpu2 cpu3
idle busy wakee idle
2. choose cpu0 like the patch proposed
no overhead but tend to make the load a little more unbalance
core0 sg core1 sg
cpu0 cpu1 cpu2 cpu3
wakee busy idle idle
May be we should add a higher scope load balance check in wake_affine(),
but that means higher overhead which is just what the patch want to
reduce...
What about some discount for sync case inside select_idle_sibling()?
For example we consider sync cpu as idle and prefer it more than the others?
Regards,
Michael Wang
>> Suggested-by: Rik van Riel <r...@redhat.com>
>> Signed-off-by: Peter Zijlstra <pet...@infradead.org>
>> ---
>> kernel/sched/fair.c | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3461,6 +3461,16 @@ select_task_rq_fair(struct task_struct *
>> if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
>> prev_cpu = cpu;
>>
>> + /*
>> + * Don't bother with select_idle_sibling() in the case of a
>> sync wakeup
>> + * where we know the only running task will soon go away. Going
>> + * through select_idle_sibling will only lead to pointless
>> ping-pong.
>> + */
>> + if (sync && prev_cpu == cpu && cpu_rq(cpu)->nr_running == 1) {
>> + new_cpu = cpu;
>> + goto unlock;
>> + }
>> +
>> new_cpu = select_idle_sibling(p, prev_cpu);
>> goto unlock;
>> }
>
>
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