linux-kernel - Re: CFS review

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Date:	Mon, 27 Aug 2007 17:46:47 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS review

Ingo Molnar wrote:
> * Al Boldi <a1426z@...ab.com> wrote:
> > > could you send the exact patch that shows what you did?
> >
> > On 2.6.22.5-v20.3 (not v20.4):
> >
> > 340-    curr->delta_exec += delta_exec;
> > 341-
> > 342-    if (unlikely(curr->delta_exec > sysctl_sched_stat_granularity))
> > { 343://          __update_curr(cfs_rq, curr);
> > 344-            curr->delta_exec = 0;
> > 345-    }
> > 346-    curr->exec_start = rq_of(cfs_rq)->clock;
>
> ouch - this produces a really broken scheduler - with this we dont do
> any run-time accounting (!).

Of course it's broken, and it's not meant as a fix, but this change allows 
you to see the amount of overhead as well as any miscalculations 
__update_curr incurs.

In terms of overhead, __update_curr incurs ~3x slowdown, and in terms of 
run-time accounting it exhibits a ~10sec task-startup miscalculation.

> Could you try the patch below instead, does this make 3x glxgears smooth
> again? (if yes, could you send me your Signed-off-by line as well.)

The task-startup stalling is still there for ~10sec.

Can you see the problem on your machine?


Thanks!

--
Al

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