Re: [PATCH V2 Resend 3/4] workqueue: Schedule work on non-idle cpuinstead of current one

[Vincent Guittot]

On 27 November 2012 16:04, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Tue, 2012-11-27 at 15:55 +0100, Vincent Guittot wrote:
>> On 27 November 2012 14:59, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> > On Tue, 2012-11-27 at 19:18 +0530, Viresh Kumar wrote:
>> >> On 27 November 2012 18:56, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> >> > A couple of things. The sched_select_cpu() is not cheap. It has a double
>> >> > loop of domains/cpus looking for a non idle cpu. If we have 1024 CPUs,
>> >> > and we are CPU 1023 and all other CPUs happen to be idle, we could be
>> >> > searching 1023 CPUs before we come up with our own.
>> >>
>> >> Not sure if you missed the first check sched_select_cpu()
>> >>
>> >> +int sched_select_cpu(unsigned int sd_flags)
>> >> +{
>> >> +       /* If Current cpu isn't idle, don't migrate anything */
>> >> +       if (!idle_cpu(cpu))
>> >> +               return cpu;
>> >>
>> >> We aren't going to search if we aren't idle.
>> >
>> > OK, we are idle, but CPU 1022 isn't. We still need a large search. But,
>> > heh we are idle we can spin. But then why go through this in the first
>> > place ;-)
>>
>> By migrating it now, it will create its activity and wake up on the
>> right CPU next time.
>>
>> If migrating on any CPUs seems a bit risky, we could restrict the
>> migration on a CPU on the same node. We can pass such contraints on
>> sched_select_cpu
>>
>
> That's assuming that the CPUs stay idle. Now if we move the work to
> another CPU and it goes idle, then it may move that again. It could end
> up being a ping pong approach.
>
> I don't think idle is a strong enough heuristic for the general case. If
> interrupts are constantly going off on a CPU that happens to be idle
> most of the time, it will constantly be moving work onto CPUs that are
> currently doing real work, and by doing so, it will be slowing those
> CPUs down.
>

I agree that idle is probably not enough but it's the heuristic that
is currently used for selecting a CPU for a timer and the timer also
uses sched_select_cpu in this series. So in order to go step by step,
a common interface has been introduced for selecting a CPU and this
function uses the same algorithm than the timer already do.
Once we agreed on an interface, the heuristic could be updated.


> -- Steve
>
>
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