linux-kernel - Re: [PATCH 5/5] sched/rt: Optimize find_lowest

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Date:	Fri, 30 Jan 2015 00:42:47 +0800
From:	Xunlei Pang <pang.xunlei@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH 5/5] sched/rt: Optimize find_lowest_rq() to select a cache
 hot cpu

On 27 January 2015 at 22:56, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 27 Jan 2015 15:21:36 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
>
>> On Mon, Jan 19, 2015 at 04:49:40AM +0000, Xunlei Pang wrote:
>> > In find_lowest_rq(), if we can't find a wake_affine cpu from
>> > sched_domain, then we can actually determine a cache hot cpu
>> > instead of simply calling "cpumask_any(lowest_mask)" which
>> > always returns the first cpu in the mask.
>> >
>> > So, we can determine the cache hot cpu during the interation of
>> > sched_domain() in passing.
>>
>> Steve, I'm not getting this. Why are we using WAKE_AFFINE here?
>>
>
> It originated from Gregory Haskins topology patches. See
>  6e1254d2c41215da27025add8900ed187bca121d

Hi Peter, Steve,

I think the responsiveness is the most important feature for RT tasks,
so I think:
response latency > cache > SMT in significance.

I was wondering if we can take the cpuidle state into account like
current find_idlest_cpu() for CFS?
cpupri_find() can be easily modified to indicate the CPUPRI_IDLE case,
then we can select
an optimal idle cpu to improve RT tasks' responsiveness. For other
cases(mostly non-idle cpu),
I think we can rely on the existent sched_domain iteraction to select
a cache-hot cpu without
caring too much about SMT.

Any comments on this?

Thanks,
Xunlei
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