linux-kernel - Re: CONFIG_NO_HZ breaks blktrace timestamps

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Message-ID: <20080111095654.GR6258@kernel.dk>
Date:	Fri, 11 Jan 2008 10:56:54 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Dillow <dillowda@...l.gov>,
	Guillaume Chazarain <guichaz@...oo.fr>,
	linux-kernel@...r.kernel.org, linux-btrace@...r.kernel.org,
	mingo@...hat.com, tglx@...utronix.de
Subject: Re: CONFIG_NO_HZ breaks blktrace timestamps

On Fri, Jan 11 2008, Ingo Molnar wrote:
> 
> * Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > > they are from the scheduler git tree (except the first debug patch), 
> > > but queued up for v2.6.25 at the moment.
> > 
> > So this means that blktrace will be broken with CONFIG_NO_HZ for 
> > 2.6.24? That's clearly a regression.
> 
> 64-bit CONFIG_NO_HZ is a new feature in v2.6.24. If it happens on 32-bit 
> too and it didnt happen in v2.6.23 32-bit then it's a regression.

If blktrace worked in 2.6.23 and it doesn't in 2.6.24 because of some
option that isn't immediately apparent, then it's a regression. Period.

> all this comes from blktrace's original decision of using sched_clock()
> :-) It's not a global timesource and it's not trivial to turn it into a
> halfways usable global timesource.

Hey, it was a high res time source and the only one easily available :)
I'm fine with using another timesource, I'll take suggestions or patches
any day!

-- 
Jens Axboe

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