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SubjectRe: [PATCH] sched: smpnice try to wakeup modification
Tim Chen wrote:
> Peter,
>
> If there is no load on this_cpu, (i.e. tl_per_task is 0), we will fail
> the "tl + target_load(cpu, idx) <= tl_per_task" check.

This isn't the case. If this_cpu is idle tl_per_task will be set to
SCHED_LOAD_SCALE (see implementation of cpu_avg_load_per_task()) and
that expression should succeed unless the value returned by
target_load(cpu, idx) is bigger than SCHED_LOAD_SCALE. This is exactly
the same as would have happened with the original code.

(BTW cpu_avg_load_per_task()'s original implementation would have had
the effect you describe but it was modified when it was realized that it
would break the code in a lot of places (not just here). The thinking
now is that if there isn't enough data available to calculate the
average load per task for a run queue then the correct value to use is
the theoretical average i.e. SCHED_LOAD_SCALE.)

> I think the
> original intention was
> to put task on this_cpu if it has no load and when there's already one
> task on cpu. This helps spread tasks out for low load condition.
>
> Thanks.
>
> Tim
>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
>
> --- linux-2.6.16-mm2-a/kernel/sched.c 2006-03-28 16:00:37.091779904
> -0800
> +++ linux-2.6.16-mm2-b/kernel/sched.c 2006-03-28 16:09:08.237074008
> -0800
> @@ -1393,7 +1393,7 @@ static int try_to_wake_up(task_t *p, uns
>
> if (this_sd->flags & SD_WAKE_AFFINE) {
> unsigned long tl = this_load;
> - unsigned long tl_per_task = cpu_avg_load_per_task(this_cpu);
> + unsigned long sl_per_task = cpu_avg_load_per_task(cpu);
>
> /*
> * If sync wakeup then subtract the (maximum possible)
> @@ -1404,7 +1404,7 @@ static int try_to_wake_up(task_t *p, uns
> tl -= current->load_weight;
>
> if ((tl <= load &&
> - tl + target_load(cpu, idx) <= tl_per_task) ||
> + tl + target_load(cpu, idx) <= sl_per_task) ||
> 100*(tl + p->load_weight) <= imbalance*load) {
> /*
> * This domain has SD_WAKE_AFFINE and

Nevertheless, in some cases, it was difficult to decide which run
queue's average load per task should be used to replace SCHED_LOAD_SCALE
and a careful review of the code by those with expertise in this area
would be appreciated as I may have made mistakes.

Peter
--
Peter Williams pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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