linux-kernel - Re: yield() in i2c non-happy paths hits BUG under -rt patch

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Date:	Thu, 19 Nov 2009 15:00:08 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Leon Woestenberg <leon.woestenberg@...il.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Sven-Thorsten Dietrich <sven@...bigcorporation.com>,
	linux-i2c@...r.kernel.org,
	rt-users <linux-rt-users@...r.kernel.org>,
	"Ben Dooks (embedded platforms)" <ben-linux@...ff.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: yield() in i2c non-happy paths hits BUG under -rt patch

Hi Peter,

On Thu, 19 Nov 2009 14:06:54 +0100, Peter Zijlstra wrote:
> On Thu, 2009-11-19 at 12:59 +0000, Alan Cox wrote:
> > > Well, I guess only people monitoring system latency would notice, as
> > > this is the only thing yield() was supposed to help with in the first
> > > place.
> > 
> > 	if (need_resched())
> > 		schedule();
> 
> aka.
> 
> 	cond_resched();

Are you saying that most calls to yield() should be replaced with calls
to cond_resched()?

I admit I a little skeptical. While the description of cond_resched()
("latency reduction via explicit rescheduling in places that are safe")
sounds promising, following the calls leads me to:

static inline int need_resched(void)
{
	return unlikely(test_thread_flag(TIF_NEED_RESCHED));
}

So apparently the condition for need_resched() to do anything is
considered unlikely... suggesting that cond_resched() is a no-op in
most cases? I don't quite get the point of moving away from sched()
because it is a no-op, if we end up with a no-op under a different name.

-- 
Jean Delvare
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