* [PATCH] sched: deduplicate atomic operations in select_nohz_load_balancer()
@ 2011-05-25 12:48 Hillf Danton
2011-05-31 20:21 ` Steven Rostedt
0 siblings, 1 reply; 2+ messages in thread
From: Hillf Danton @ 2011-05-25 12:48 UTC (permalink / raw)
To: LKML
Cc: Ingo Molnar, Peter Zijlstra, Steven Rostedt, Mike Galbraith, Yong Zhang
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);
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);
- 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)) {
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;
}
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] sched: deduplicate atomic operations in select_nohz_load_balancer()
2011-05-25 12:48 [PATCH] sched: deduplicate atomic operations in select_nohz_load_balancer() Hillf Danton
@ 2011-05-31 20:21 ` Steven Rostedt
0 siblings, 0 replies; 2+ messages in thread
From: Steven Rostedt @ 2011-05-31 20:21 UTC (permalink / raw)
To: Hillf Danton
Cc: LKML, Ingo Molnar, Peter Zijlstra, Mike Galbraith, Yong Zhang
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
^ permalink raw reply [flat|nested] 2+ messages in thread
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2011-05-25 12:48 [PATCH] sched: deduplicate atomic operations in select_nohz_load_balancer() Hillf Danton
2011-05-31 20:21 ` Steven Rostedt
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