linux-kernel - Re: [PATCH] virtual sched

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Date:	Thu, 19 Jul 2007 18:00:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Jan Glauber <jang@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, vatsa@...ux.vnet.ibm.com,
	mschwid2@...ux.vnet.ibm.com, efault@....de,
	dmitry.adamushko@...il.com, paulus@...ba.org, anton@...ba.org
Subject: Re: [PATCH] virtual sched_clock() for s390


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> >  /*
> > - * Monotonic_clock - returns # of nanoseconds passed since time_init()
> > + * Scheduler clock - returns current time in nanosec units.
> > + * Now based on virtual cpu time to only account time the guest
> > + * was actually running.
> 
> Runn*ing*?  Does it include time the VCPU spends idle/blocked?  If 
> not, then the scheduler won't be able to tell how long a process has 
> been asleep.  Maybe this doesn't matter (I had this problem in a 
> version of Xen's sched_clock, and I can't say I saw an ill effects 
> from it).

CFS does measure time elapsed across task-sleep periods (and does 
something similar to what the old scheduler's 'sleep average' 
interactivity mechanism did), but that mechanism measures "time spent 
running during sleep", not "time spent idling".

still, CFS needs time measurement across idle periods as well, for 
another purpose: to be able to do precise task statistics for /proc. 
(for top, ps, etc.) So it's still true that sched_clock() should include 
idle periods too.

	Ingo
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