Re: [PATCH] Extend mwait idle to optimize away CAL and RES interrupts to an idle CPU -v1

From: Venki Pallipadi <venki@google.com>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Aaron Durbin <adurbin@google.com>, Paul Turner <pjt@google.com>,
	Yong Zhang <yong.zhang0@gmail.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Extend mwait idle to optimize away CAL and RES interrupts to an idle CPU -v1
Date: Thu, 1 Mar 2012 18:00:51 -0800	[thread overview]
Message-ID: <CABeCy1a7RszUWBV10tVbdsxAdPpQAMSQXw-pMG2znTqYb5WN5A@mail.gmail.com> (raw)
In-Reply-To: <1330652258.30167.70.camel@sbsiddha-desk.sc.intel.com>

On Thu, Mar 1, 2012 at 5:37 PM, Suresh Siddha <suresh.b.siddha@intel.com> wrote:
> On Thu, 2012-03-01 at 17:35 -0800, Venki Pallipadi wrote:
>> On Thu, Mar 1, 2012 at 5:28 PM, Suresh Siddha <suresh.b.siddha@intel.com> wrote:
>> > On Thu, 2012-03-01 at 16:33 -0800, Venki Pallipadi wrote:
>> >> >
>> >> > fork_idle() should also make sure it does not schedule the child
>> >> > thread: thus we'd also be able to further simplify smpboot.c and
>> >> > get rid of all that extremely ugly 'struct create_idle'
>> >> > gymnastics in smpboot.c.
>> >>
>> >> But not this. I am not sure where fork_idle results in resched of the child.
>> >> As I saw it, fork_idle calls init_idle and that sets the affinity of
>> >> idle_task to target CPU. So, reschedule should not be a problem. What
>> >> am I missing here?
>> >
>> > I think Ingo is referring to the fact that we can't use kthread_create()
>> > here and hence we were relying on fork_idle().
>> >
>> >> Also, I tried this silly test patch (Cut and paste... Sorry) and it
>> >> seemed to work fine both with and without CPU hotplug.
>> >>
>> >
>> > I don't think we can do this today, as we need to make sure we have the
>> > correct current context. With dynamic cpu hotplug, current context can
>> > be any process and hence we were depending on the schedule_work()
>> > context.
>> >
>>
>> schedule_work() is only done at boot time. In case of dynamic cpu
>> hotplug, we skip the whole fork_idle as we already have the task
>> struct and just do init_idle().
>>
>
> What happens if we boot with "maxcpus=" and later online the remaining
> cpu's?

Yes. This case will be problematic. So, we still need struct
create_idle work stuff even after percpu idle_task. Or was Ingo's
suggestion to do something along the lines of - init any CPU's idle
task from CPU 0's idle task?

> same issue with the physical cpu-online case too right?
Any fresh online at runtime. Yes.

>
> thanks,
> suresh
>