linux-kernel - Re: [RFC][PATCH 1/9] sched: Introduce power scheduler

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Date:	Wed, 10 Jul 2013 13:19:18 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"alex.shi@...el.com" <alex.shi@...el.com>,
	"efault@....de" <efault@....de>, "pjt@...gle.com" <pjt@...gle.com>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"corbet@....net" <corbet@....net>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC][PATCH 1/9] sched: Introduce power scheduler

On 10 July 2013 13:11, Morten Rasmussen <morten.rasmussen@....com> wrote:
> On Wed, Jul 10, 2013 at 03:10:15AM +0100, Arjan van de Ven wrote:
>> On 7/9/2013 8:55 AM, Morten Rasmussen wrote:
>> > +   mod_delayed_work_on(schedule_cpu(), system_wq, &dwork,
>> > +                           msecs_to_jiffies(INTERVAL));
>>
>> so thinking about this more, this really really should not be a work queue.
>> a work queue will cause a large number of context switches for no reason
>> (on Intel and AMD you can switch P state from interrupt context, and I'm pretty sure
>> that holds for many ARM as well)
>
> Agree. I should have made it clear this is only a temporary solution. I
> would prefer to tie the power scheduler to the existing scheduler tick
> instead so we don't wake up cpus unnecessarily. nohz may be able handle
> that for us. Also, currently the power scheduler updates all cpus.
> Going forward this would change to per cpu updates and partial updates
> of the global view to improve scalability.

For the packing tasks patches, we are using the periodic load balance
sequence to update the activity like it is done for the cpu_power. I
have planned to update the packing patches to see how it can cooperate
with Morten patches as it has similar needs.

>
>>
>> and in addition, it causes some really nasty cases, especially around real time tasks.
>> Your workqueue will schedule a kernel thread, which will run
>> BEHIND real time tasks, and such real time task will then never be able to start running at a higher performance.
>>
>> (and with the delta between lowest and highest performance sometimes being 10x or more,
>> the real time task will be running SLOW... quite possible longer than several milliseconds)
>>
>> and all for no good reason; a normal timer running in irq context would be much better for this kind of thing!
>>
>>
>
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