Hey Tejun, On 27 April 2013 00:41, Tejun Heo <tj@xxxxxxxxxx> wrote: > Hey, Viresh. > > It's already too late for the upcoming merge window, but things > generally look good to me and I'll apply the patchset once wq/for-3.11 > opens. One nitpick tho. Obviously. I understand this and agree with you on it. It should go in 3.11 now. > On Thu, Apr 25, 2013 at 09:13:44AM +0530, Viresh Kumar wrote: >> + workqueue.power_efficient >> + Workqueues can be performance or power-oriented. >> + Currently, most workqueues are bound to the CPU they > ^^^^ > per-cpu would be better > >> + were created on. This gives good performance (due to >> + cache effects) at the cost of potentially waking up >> + otherwise idle cores just to process some work. To save >> + power, we can allow the work to be rescheduled on a core >> + that is already awake. > > The above description is confusing to me. As have been discussed > multiple times before, per-cpu workqueue in itself doesn't wake up the > CPU physically. The timer may but per-cpu workqueue doesn't. It was > confusing when this patchset was first posted and the above phrasing > is still confusing. What the patchset tries to do is preventing the > scheduler from perceiving the CPU as active due to the activated > worker thread pinned to that CPU, right? The knob doesn't really do > anything about waking up the processor in itself. It just avoids > feeding the scheduler with noisy activation events and allows it to > allocate work item execution according to the scheduler's view of CPU > active/idleness. As the scheduler has longer / larger scope of > overall CPU activities and means to regulate them, this leads to more > power-efficient allocation of work item executions, right? It'd be > really great if the descriptions and the comment above the flag makes > this abundantly clear because it's not something too apparent. Whatever you wrote above confused me even more :) This is what i had in my mind until now. Its not about per-cpu workqueue. Lets take example of system_wq. It doesn't have WQ_UNBOUND flag set. Now if we call queue_work_on() with cpu x and sytem_wq, then work will execute on cpu x. If we call queue_work() then it will queue the work on local cpu. 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. - 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. And what timer are you talking about? I am not talking about deffered work only, but normal work too. I might have wrongly phrased some part of my patch (maybe used workqueue instead of work), will fix that up. -- viresh -- 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
