linux-kernel - Re: [PATCH] sched: fix conflict of schedule domain balance in RT scheduling

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Message-ID: <BANLkTik60sZNK6=CXte57O8Cs4ohU_fvVA@mail.gmail.com>
Date:	Fri, 3 Jun 2011 23:21:57 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Yong Zhang <yong.zhang0@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: fix conflict of schedule domain balance in RT scheduling

On Fri, Jun 3, 2011 at 10:52 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 2011-06-03 at 22:30 +0800, Hillf Danton wrote:
>
>> I note the comment related to cache-hot, it is fine for woken task even though
>> cache-hot is considered to be nop for RT task,
>
> I would not say cache-hot is a nop for RT tasks. Although RT tasks
> should not depend on cache being hot, we don't want to go out of our way
> to hurt RT tasks.
>
>
>>  but what dose it help pushing
>> RT tasks waiting on RQ?
>
> What the lowest_mask returns is a mask of all cpus with the lowest
> priority. Due to the double_lock_balance() we could have unlocked the rq
> lock, and things could have changed. Thus, the current CPU could have
> lowered its priority. Unlikely to be the case, but can happen.
>

I am not sure task switch could happen due to RQ unlock.
If no switch, how priority is lowered?

thanks
            Hillf


> If the lowest_mask has the task's CPU set, why push this RT task
> somewhere else? Most likely the current CPU will not be set, but lets
> not punish the RT task if it is.
>
> -- Steve
>
>
>
>
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