linux-kernel - Re: [PATCH] sched: Support current clocksource handling in fallback sched

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Message-ID: <20090527002615.GA8845@linux-sh.org>
Date:	Wed, 27 May 2009 09:26:15 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	john stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk
Subject: Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock().

On Tue, May 26, 2009 at 05:22:10PM -0700, john stultz wrote:
> On Wed, 2009-05-27 at 08:44 +0900, Paul Mundt wrote:
> > What would you recommend instead? We do not want to spin here, and if we
> > are in the middle of changing clocksources and returning jiffies anyways,
> > then this same issue pops up in the current sched_clock() implementation
> > regardless of whether we are testing for lock contention or not.
> > Likewise, even if we were to spin, the same situation exists if the new
> > clocksource does not have the _SCHED_CLOCK bit set and we have to fall
> > back on jiffies anyways, doesn't it?
> > 
> > Put another way, and unless I'm missing something obvious, if we ignore
> > my changes to sched_clock(), how is your concern not applicable to case
> > where we are changing clocksources and using generic sched_clock() as it
> > is today?
> 
> Well, Thomas' point that locking isn't necessary, as sched_clock()
> doesn't have to be correct, is probably right. 
> 
> So, I think a get_sched_clocksource() interface would be ideal (if we
> want to get academic at a later date, the pointer could be atomically
> updated, and we'd keep it valid for some time via an rcu like method).
> 
> Additionally, you can set the jiffies clocksource as a _SCHED_CLOCK
> clocksource and drop the jiffies fallback code completely.
> 
I thought about that initially as well, but in the case of the jiffies
clocksource, that won't handle INITIAL_JIFFIES, which we want to subtract
to make printk times sane.
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