linux-kernel - Re: Q: sched_clock() vs. clocksource, how to implement correctly

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Message-ID: <1272325694.32741.16.camel@c-dwalke-linux.qualcomm.com>
Date:	Mon, 26 Apr 2010 16:48:14 -0700
From:	Daniel Walker <dwalker@...o99.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Johannes Stezenbach <js@...21.net>,
	John Stultz <johnstul@...ibm.com>,
	linux-kernel@...r.kernel.org, Magnus Damm <damm@...nsource.se>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Q: sched_clock() vs. clocksource, how to implement correctly

On Fri, 2010-04-23 at 18:29 +0200, Martin Schwidefsky wrote:
> > Questions:
> > 
> > - Isn't sched_clock() supposed to be extended to 64bit so
> >   that it practically never wraps?
> >   (old implementations use cnt32_to_63())
> 
> Yes, sched_clock() is supposed to return a monotonic timestamp.
>  
> > - What would be the effect on scheduling when sched_clock() wraps?
> 
> It would confuse the process accounting and the scheduling I guess.
> 

Are you sure about this? I'm pretty sure I've seen Ingo say multiple
times that sched_clock can wrap, and can be unstable. For instance
sched_clock is (was?) directly connected to the TSC on x86 ..

If it really can't wrap there must bunches of architectures that would
need to be fixed up.

Daniel

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