linux-kernel - Re: Stolen and degraded time and schedulers

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Date:	Wed, 14 Mar 2007 14:18:39 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Dan Hecht <dhecht@...are.com>
CC:	dwalker@...sta.com, cpufreq@...ts.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Con Kolivas <kernel@...ivas.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	john stultz <johnstul@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>, paulus@...ibm.com,
	schwidefsky@...ibm.com, Rik van Riel <riel@...hat.com>
Subject: Re: Stolen and degraded time and schedulers

Dan Hecht wrote:
> On 03/14/2007 01:31 PM, Jeremy Fitzhardinge wrote:
>> Dan Hecht wrote:
>>> Sounds good.  I don't see this in your patchset you sent yesterday
>>> though; did you add it after sending out those patches?
>>
>> Yes.
>>
>>>   if so, could you forward the new patch?  does it explicitly prevent
>>> stolen time from getting accounted as  user/system time or does it
>>> just rely on NO_HZ mode sort of happening to work that way (since the
>>> one shot timer is skipped ahead for missed ticks)?
>>
>> Hm, not sure.  It doesn't care how often it gets called; it just
>> accumulates results up to that point, but I'm not sure if the time would
>> get double accounted.  Perhaps it doesn't matter when using
>> xen_sched_clock().
>>
>
> I think you might be double counting time in some cases. 
> sched_clock() isn't really relevant to stolen time accounting (i.e.
> cpustat->steal).
>
> I think what you want is to make sure that the sum of the cputime
> passed to all of:
>
> account_user_time
> account_system_time
> account_steal_time
>
> adds up to the total amount of time that has passed.  I think it is
> sort of working for you (i.e. doesn't always double count stolen
> ticks) since in NO_HZ mode, update_process_time (which calls
> account_user_time & account_system_time) happens to be skipped during
> periods of stolen time due to the hrtimer_forward()'ing of the one
> shot expiry. 

OK, this will need a closer look.

BTW, what are the properties of the vmi read_available_cycles()?  Is
that a per-cpu timer?  If its used as the timebase for sched_clock, how
does recalc_task_prio not get a -ve sleep time?


    J
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