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

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Message-ID: <BANLkTim-tr+LAg1pA-wEFvVGiCvOYdfDoQ@mail.gmail.com>
Date:	Fri, 3 Jun 2011 22:30:33 +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 10:19 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 2011-06-01 at 21:58 +0800, Hillf Danton wrote:
>
>> And checking for cache-hot is also added to confirm the comment there.
>
> Please read that comment.
>
>>       cpu = task_cpu(p);
>> @@ -1253,7 +1253,8 @@ static int find_lowest_rq(struct task_struct *task)
>>        * We prioritize the last cpu that the task executed on since
>>        * it is most likely cache-hot in that location.
>>        */
>> -     if (cpumask_test_cpu(cpu, lowest_mask))
>> +     if (cpumask_test_cpu(cpu, lowest_mask) &&
>> +         task_hot(task, task_rq(task)->clock_task, NULL))
>>               return cpu;
>
> What task_hot() checks for and what we are assuming are two different
> things. In fact, we can disable task_hot() so it always fails. That's
> not what we want.
>
> If the task happens to have ran on a CPU that is in the lowest_mask, we
> want that CPU. Time may not matter. If we know a task ran on a
> particular CPU last, we want to run it there if possible. It may still
> have cache lines for it, even if it has been a long time since it last
> ran.
>
Hi Steve

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, but what dose it help pushing
RT tasks waiting on RQ?

thanks
           Hillf
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