linux-kernel - Re: [RFC][PATCH 1/7] sched: Replace post_schedule with a balance callback list

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Message-ID: <20150603085527.GA3644@twins.programming.kicks-ass.net>
Date:	Wed, 3 Jun 2015 10:55:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	pang.xunlei@....com.cn
Cc:	juri.lelli@...il.com, ktkhai@...allels.com,
	linux-kernel@...r.kernel.org, mingo@...e.hu, oleg@...hat.com,
	pang.xunlei@...aro.org, rostedt@...dmis.org,
	umgwanakikbuti@...il.com
Subject: Re: [RFC][PATCH 1/7] sched: Replace post_schedule with a balance
 callback list

A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

On Wed, Jun 03, 2015 at 04:24:05PM +0800, pang.xunlei@....com.cn wrote:
> Hi Peter,
> 
> This may increase the overhead of schedule() a bit, as it will have 
> more work to do.

How so? It replaces the post_schedule() muck and should not be more
expensive than that.

It will make sched_setscheduler() etc.. a little more expensive, but
that doesn't matter, those are not critical things at all.

> check_class_changed():
>         if (prev_class->switched_from)
>                         prev_class->switched_from(rq, p);
>                 /* Possble rq->lock 'hole'.  */
>                 p->sched_class->switched_to(rq, p);
> 
> For above cases, why can't we just add a judgement in switched_to_fair() 
> as follows:
> if (rq != task_rq(p))
>         return;

Because its too easy to get wrong. There have been many instances of
bugs caused by this dropping of rq->lock.

And sure you can patch it up, once you find it, but I would really
rather prevent these things if at all possible.
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