linux-kernel - Re: [PATCH v2 2/6] sched/rt: Optimize select_task_rq

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Message-ID: <CADcy93Xeb8+pkyjPCazpA5_fhZycrYNf654Ev6j2tEkwqdJXmg@mail.gmail.com>
Date:	Tue, 4 Nov 2014 23:09:09 +0800
From:	"pang.xunlei" <pang.xunlei@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH v2 2/6] sched/rt: Optimize select_task_rq_rt() for non-RT
 curr task

On 4 November 2014 22:47, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 4 Nov 2014 22:29:24 +0800
> "pang.xunlei" <pang.xunlei@...aro.org> wrote:
>
>
>> > Migration is not cheap. It causes cache misses and TLB flushes. This is
>> > not something that should be taken lightly.
>> Ok, thanks!
>> But I think the PUSH operation optimized by the former patch is reasonable,
>> since PUSH itselft does involve the Migration. Do I miss something?
>
> For the first patch you may be right, but I want to think about it some
> more. I want to make sure we are not adding any other type of overhead
> with the extra calls.
Yes, this may cause some overhead/latency in idle especially its exit
stage, if that can't be accepted, I think it can also be done just in
find_lowest_rq() after cpupri_find(), we can modify cpupri_find() for
example to return a pri_to_cpu[] index plus one instead of 1, then if
the return index equals CPUPRI_NORMAL+1, then iterate the
"lowest_mask" with something like cpu_idle() judgement to select the
idle cpu.

>
> -- Steve
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