On Thu, 2012-09-27 at 12:40 -0700, Linus Torvalds wrote: > On Thu, Sep 27, 2012 at 11:29 AM, Peter Zijlstra <a.p.zijls...@chello.nl> > wrote: > > > > Don't forget to run the desktop interactivity benchmarks after you're > > done wriggling with this knob... wakeup preemption is important for most > > those. > > So I don't think we want to *just* wiggle that knob per se. We > definitely don't want to hurt latency on actual interactive asks. But > it's interesting that it helps psql so much, and that there seems to > be some interaction with the select_idle_sibling(). > > So I do have a few things I react to when looking at that wakeup granularity.. > > I wonder about this comment, for example: > > * By using 'se' instead of 'curr' we penalize light tasks, so > * they get preempted easier. That is, if 'se' < 'curr' then > * the resulting gran will be larger, therefore penalizing the > * lighter, if otoh 'se' > 'curr' then the resulting gran will > * be smaller, again penalizing the lighter task. > > why would we want to preempt light tasks easier? It sounds backwards > to me. If they are light, we have *less* reason to preempt them, since > they are more likely to just go to sleep on their own, no?
At, that particular 'light' refers to se->load.weight. > Another question is whether the fact that this same load interacts > with select_idle_sibling() is perhaps a sign that maybe the preemption > logic is all fine, but it interacts badly with the "pick new cpu" > code. In particular, after having changed rq's, is the vruntime really > comparable? IOW, maybe this is an interaction between "place_entity()" > and then the immediately following (?) call to check wakeup > preemption? I think vruntime should be fine. We set take the delta between the task's vruntime when it went to sleep and it's previous rq min_vruntime to capture progress made while it slept, and apply the relative offset in the task's new home so a task can migrate and still have a chance to preempt on wakeup. > The fact that *either* changing select_idle_sibling() *or* changing > the wakeup preemption granularity seems to have such a huge impact > does seem to tie them together somehow for this particular load. No? The way I read it, Boris had wakeup preemption disabled. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
