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 <[email protected]>
--- 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 [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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