On Wed, 2015-05-27 at 17:22 -0400, Josef Bacik wrote:
> [ sorry if you get this twice, it seems like the first submission got lost ]
>
> At Facebook we have a pretty heavily multi-threaded application that is
> sensitive to latency. We have been pulling forward the old SD_WAKE_IDLE code
> because it gives us a pretty significant performance gain (like 20%). It
> turns
> out this is because there are cases where the scheduler puts our task on a
> busy
> CPU when there are idle CPU's in the system. We verify this by reading the
> cpu_delay_req_avg_us from the scheduler netlink stuff. With our crappy patch
> we
> get much lower numbers vs baseline.
>
> SD_BALANCE_WAKE is supposed to find us an idle cpu to run on, however it is
> just
> looking for an idle sibling, preferring affinity over all else. This is not
> helpful in all cases, and SD_BALANCE_WAKE's job is to find us an idle cpu, not
> garuntee affinity. Fix this by first trying to find an idle sibling, and then
> if the cpu is not idle fall through to the logic to find an idle cpu. With
> this
> patch we get slightly better performance than with our forward port of
> SD_WAKE_IDLE. Thanks,
The job description isn't really find idle. it's find least loaded.
> Signed-off-by: Josef Bacik <jba...@fb.com>
> Acked-by: Rik van Riel <r...@redhat.com>
> ---
> kernel/sched/fair.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 241213b..03dafa3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4766,7 +4766,8 @@ select_task_rq_fair(struct task_struct *p, int
> prev_cpu, int sd_flag, int wake_f
>
> if (sd_flag & SD_BALANCE_WAKE) {
> new_cpu = select_idle_sibling(p, prev_cpu);
> - goto unlock;
> + if (idle_cpu(new_cpu))
> + goto unlock;
> }
>
> while (sd) {
Instead of doing what for most will be a redundant idle_cpu() call,
perhaps a couple cycles can be saved if you move the sd assignment above
affine_sd assignment, and say if (!sd || idle_cpu(new_cpu)) ?
You could also stop find_idlest_group() at the first completely idle
group to shave cycles off the not fully committed search. It ain't
likely to find a negative load.. cool as that would be ;-)
-Mike
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