Messages in this thread | | | Date | Sun, 3 Jul 2011 14:07:21 +0600 | Subject | Re: [PATCH] sched: Check nr_running before calling pick_next_task in schedule(). | From | Rakib Mullick <> |
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On Sat, Jul 2, 2011 at 9:35 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Sat, 2011-07-02 at 20:26 +0600, Rakib Mullick wrote: >> Well, yes - branching seems definitely have some side effects. > > It adds the cost of the test as well as a possible branch mis-predict. > >> Thinking from UP's perspective, it will only hit slow path -- going >> into idle. > > Uhm, no, every time the machine is busy and does a schedule between > tasks you still get to do that extra nr_running test and branch. > Ok, for now I'm putting branch aside. I don't think checking nr_running isn't extra, it should be norm. Cause, if this rq has no nr_running it calls idle_balance, at that point due to idle_balance - we might have moved task from other rq if idle_balance is successful. But, idle_balance might not be successful that's why I was thinking about checking nr_running is necessary, in that case we don't need to call pick_next_task cause - we don't have any task.
>> In that case, that likely branch will just fail. And on an >> UP system that slow path -- going into idle -- is the only way, taking >> the fast path (trying picking a task) isn't the right thing, isn't >> it? > > I'm not at all sure I even understand what you're trying to say. I > really don't understand what's the problem with going the long way with > picking the idle task, the machine is idle, it doesn't have anything > useful to do, who cares. > Well, yes the machine is idle. I got your point that you're emphasizing that CPU is idle even if we take long path it doesn't matter. But, when we've two ways, one is going through pick_next_task other is calling idle_class straight I think calling idle_class is better. Actually that's how I think (and certainly it differs from yours). Note that, in pick_next_task there is branch, which checks likely(nr_running==cfs->nr_running) - chances for hitting this branch will increase - cause in case of !nr_running, pick_next_task won't be called. It will reduce pick_next_task's calling overhead.
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