linux-kernel - Re: delayacct regression in 2.6.24 git

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Date:	Fri, 02 Nov 2007 21:58:14 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: delayacct regression in 2.6.24 git

Ingo Molnar wrote:
> * Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
>> Hi, Ingo,
>>
>> I missed commit 75d4ef16a6aa84f708188bada182315f80aab6fa
>> "sched: fix delay accounting performance regression"
>>
>> Without these changes, I find that delay accounting does
>> not work as expected, I see zeros in the field for virtual,delay
>> and count(s).
>>
>> CPU             count     real total  virtual total    delay total
>>                     0      535861848              0              0
>>
>>
>> This breaks existing delay accounting behaviour.
>>
>> Am I missing something?
> 
> delay accounting was causing context-switch slowdown because it called 
> sched_clock() explicitly - but indeed my fix to that is wrong ...
> 
> how about reverting the change but also using rq->clock instead of 
> sched_clock()?

Hmm... I think that might work, I find that schedule() calls
__update_rq_clock() which in turn calls sched_clock(), so it
might be a worthwhile optimization.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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