Hi Vincent & Peter,
On 28/05/14 07:49, Vincent Guittot wrote: [...]
Nick,
While doing some rework on the wake affine part of the scheduler, i failed to catch the use case that takes advantage of a condition that you added some while ago with the commit a3f21bce1fefdf92a4d1705e888d390b10f3ac6f
Could you help us to clarify the 2 first lines of the test that you added ?
if ((tl <= load &&tl + target_load(cpu, idx) <=SCHED_LOAD_SCALE) ||
100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {Regards, Vincent
commit a3f21bce1fefdf92a4d1705e888d390b10f3ac6f
if ((tl <= load &&tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {So back when the code got introduced, it read:
target_load(prev_cpu, idx) - sync*SCHED_LOAD_SCALE < source_load(this_cpu, idx) && target_load(prev_cpu, idx) - sync*SCHED_LOAD_SCALE + target_load(this_cpu, idx) < SCHED_LOAD_SCALE
Shouldn't this be
target_load(this_cpu, idx) - sync*SCHED_LOAD_SCALE <= source_load(prev_cpu, idx) && target_load(this_cpu, idx) - sync*SCHED_LOAD_SCALE + target_load(prev_cpu, idx) <= SCHED_LOAD_SCALE
"[PATCH] sched: implement smpnice" (2dd73a4f09beacadde827a032cf15fd8b1fa3d48) mentions that SCHED_LOAD_BALANCE (IMHO, should be SCHED_LOAD_SCALE) represents the load contribution of a single task. So I read the second part as if the sum of the load of this_cpu and prev_cpu is smaller or equal to the (maximal) load contribution (maximal possible effect) of a single task.
There is even a comment in "[PATCH] sched: tweak affine wakeups" (a3f21bce1fefdf92a4d1705e888d390b10f3ac6f) in try_to_wake_up() when SCHED_LOAD_SCALE gets subtracted from tl = this_load = target_load(this_cpu, idx):
+ * If sync wakeup then subtract the (maximum possible) + * effect of the currently running task from the load + * of the current CPU:
"[PATCH] sched: implement smpnice" then replaces SCHED_LOAD_SCALE w/
+static inline unsigned long cpu_avg_load_per_task(int cpu) +{ + runqueue_t *rq = cpu_rq(cpu); + unsigned long n = rq->nr_running; + + return n ? rq->raw_weighted_load / n : SCHED_LOAD_SCALE;
-- Dietmar
So while the first line makes some sense, the second line is still somewhat challenging.
I read the second line something like: if there's less than one full task running on the combined cpus.
ok. your explanation makes sense
Maybe, its still slightly weird :-)
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