Re: [PATCH V5 1/5] workqueues: Introduce new flag WQ_POWER_EFFICIENT for power oriented workqueues

[Viresh Kumar <viresh.kumar@linaro.org>]

On 29 April 2013 21:49, Tejun Heo <tj@kernel.org> 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.