Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 27 May 2014 17:19:02 +0200 | Subject | Re: [PATCH v2 02/11] sched: remove a wake_affine condition |
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On 27 May 2014 14:48, Peter Zijlstra <peterz@infradead.org> wrote: > On Fri, May 23, 2014 at 05:52:56PM +0200, Vincent Guittot wrote: >> I have tried to understand the meaning of the condition : >> (this_load <= load && >> this_load + target_load(prev_cpu, idx) <= tl_per_task) >> but i failed to find a use case that can take advantage of it and i haven't >> found description of it in the previous commits' log. > > commit 2dd73a4f09beacadde827a032cf15fd8b1fa3d48 > > int try_to_wake_up(): > > in this function the value SCHED_LOAD_BALANCE is used to represent the load > contribution of a single task in various calculations in the code that > decides which CPU to put the waking task on. While this would be a valid > on a system where the nice values for the runnable tasks were distributed > evenly around zero it will lead to anomalous load balancing if the > distribution is skewed in either direction. To overcome this problem > SCHED_LOAD_SCALE has been replaced by the load_weight for the relevant task > or by the average load_weight per task for the queue in question (as > appropriate). > > if ((tl <= load && > - tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) || > - 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) { > + tl + target_load(cpu, idx) <= tl_per_task) || > + 100*(tl + p->load_weight) <= imbalance*load) {
The oldest patch i had found was: https://lkml.org/lkml/2005/2/24/34 where task_hot had been replaced by + if ((tl <= load && + tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) || + 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
but as explained, i haven't found a clear explanation of this condition
> > > 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 > > 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
> > Now for idx==0 this is hard, because even when sync=1 you can only make > it true if both cpus are completely idle, in which case you really want > to move to the waking cpu I suppose.
This use case is already taken into account by
if (this_load > 0) .. else balance = true
> > One task running will have it == SCHED_LOAD_SCALE. > > But for idx>0 this can trigger in all kinds of situations of light load.
target_load is the max between load for idx == 0 and load for the selected idx so we have even less chance to match the condition : both cpu are completely idle
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