Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE

[Mike Galbraith <umgwanakikbuti@gmail.com>]

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 <jbacik@fb.com>
> Acked-by: Rik van Riel <riel@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