On 29 April 2013 21:49, Tejun Heo <tj@xxxxxxxxxx> wrote: > On Mon, Apr 29, 2013 at 12:06:28PM +0530, Viresh Kumar wrote: > Yeap, !WQ_UNBOUND workqueues == per-cpu workqueues. Sigh!! You were talking about thread per cpu here... Sorry for missing it earlier :( >> At this time local cpu may be busy or idle (Atleast according to scheduler). >> We don't want a idle cpu (From schedulers perspective) to be used for >> running this work's handler due to two reasons. >> - idle cpu may be in WFI or deeper idle states and so we can avoid waking >> it up. > > I have no idea what WFI is but the physical CPU is already awake at > that time. It can't be idle - it's running queue_work(). It could be > running in lower freq tho, which each code piece doesn't really have > much control over. Stupid point. WFI: Wait for interrupt (low power mode of cpu). >> - We will make idle cpu look busy and so other kernel stuff may be scheduled >> on it now. But we could have kept it idle for a long time. > > Hmmm... yeah, about the same thing I wrote, it's not really about not > waking up the CPU right now physically but avoiding forcing the > scheduler scheduling a pinned task on an otherwise quiescent CPU. > This effectively allows the scheduler to migrate such work items > towards a CPU which the scheduler considers to be better (in power or > whatever) leading to noticeable powersave. Correct. >> And what timer are you talking about? I am not talking about deffered work only, >> but normal work too. > > Deferred work item == timer + work item. Ya, i knew that :) >> I might have wrongly phrased some part of my patch (maybe used workqueue >> instead of work), will fix that up. > > I think it'd be necessary to distinguish the physical CPU being idle > and the scheduler considers it to be idle (no task to schedule on it) > and explain how increasing the latter can lead to powersave. As it's > currently written, it seemingly, to me anyway, suggests that the > proposed change somehow avoids waking up actually idle CPU, which > isn't the case as queue_work() *always* schedules on the local CPU. > The local CPU can't be idle by definition. Yes you are correct. I will fix it. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
