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SubjectRe: [PATCH] sched: deduplicate atomic operations in select_nohz_load_balancer()
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On Wed, 2011-05-25 at 20:48 +0800, Hillf Danton wrote:
> Two atomic-related operations, atomic_read and atomic_cmpxchg, are
> used here to do the job. Since comparison is already included in the
> later, the former is folded in the later, and operation is shorter.
>
> Signed-off-by: Hillf Danton <dhillf@gmail.com>
> ---
>
> --- tip-git/kernel/sched_fair.c Sat May 14 15:21:56 2011
> +++ sched_fair.c Wed May 25 20:15:34 2011
> @@ -3806,35 +3806,25 @@ void select_nohz_load_balancer(int stop_
>
> if (stop_tick) {
> if (!cpu_active(cpu)) {
> - if (atomic_read(&nohz.load_balancer) != cpu)
> - return;
> -
> /*
> * If we are going offline and still the leader,
> * give up!
> */
> - if (atomic_cmpxchg(&nohz.load_balancer, cpu,
> - nr_cpu_ids) != cpu)
> - BUG();
> + atomic_cmpxchg(&nohz.load_balancer, cpu, nr_cpu_ids);

OK, I can handle this change.

>
> return;
> }
>
> cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
>
> - if (atomic_read(&nohz.first_pick_cpu) == cpu)
> - atomic_cmpxchg(&nohz.first_pick_cpu, cpu, nr_cpu_ids);
> - if (atomic_read(&nohz.second_pick_cpu) == cpu)
> - atomic_cmpxchg(&nohz.second_pick_cpu, cpu, nr_cpu_ids);
> + atomic_cmpxchg(&nohz.first_pick_cpu, cpu, nr_cpu_ids);
> + atomic_cmpxchg(&nohz.second_pick_cpu, cpu, nr_cpu_ids);

This too, but we are doing a lot of cmpxchg's here when we probably
don't need to. But this does not look like a fast path so it may not be
that big of a deal.

>
> - if (atomic_read(&nohz.load_balancer) >= nr_cpu_ids) {
> + /* make me the ilb owner */
> + if (nr_cpu_ids ==
> + atomic_cmpxchg(&nohz.load_balancer, nr_cpu_ids, cpu)) {

Hmm, I see what you are doing here. But to fit the rest of the code, it
should be:

if (atomic_cmpxchg(&nohz.load_balancer, nr_cpu_ids, cpu) ==
nr_cpu_ids) {



> int new_ilb;
>
> - /* make me the ilb owner */
> - if (atomic_cmpxchg(&nohz.load_balancer, nr_cpu_ids,
> - cpu) != nr_cpu_ids)
> - return;
> -
> /*
> * Check to see if there is a more power-efficient
> * ilb.
> @@ -3853,10 +3843,7 @@ void select_nohz_load_balancer(int stop_
>
> cpumask_clear_cpu(cpu, nohz.idle_cpus_mask);
>
> - if (atomic_read(&nohz.load_balancer) == cpu)
> - if (atomic_cmpxchg(&nohz.load_balancer, cpu,
> - nr_cpu_ids) != cpu)
> - BUG();
> + atomic_cmpxchg(&nohz.load_balancer, cpu, nr_cpu_ids);
> }
> return;
> }

The only other thing I can see that might be of issue, but not a big
one, is that we are removing BUG checks. We may have been able to detect
a bug before by seeing that the load_balancer was one thing, and
suddenly changed when it shouldn't have. This may not be a big deal, but
I just wanted to bring that up.

This change needs to be acked by Peter. I don't work much with the
sched_fair code.

-- Steve




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