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

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Date:	Thu, 19 Nov 2009 14:11:18 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jean Delvare <khali@...ux-fr.org>
cc:	Leon Woestenberg <leon.woestenberg@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: yield() in i2c non-happy paths hits BUG under -rt patch

On Thu, 19 Nov 2009, Jean Delvare wrote:
> > That still does not explain why yield() is necessary _between_ the
> > transaction attempts.
> 
> It is not _necessary_. We are just trying to be fair to other kernel
> threads, because bit-banging is expensive and this is the only case
> where we know we can tolerate a delay.
> 
> Just to clarify things... does (or did) yield() have anything to do
> with CONFIG_PREEMPT_VOLUNTARY?

No.
 
> > That code is fully preemptible, otherwise you could not call
> > yield().
> 
> How does one know what code is preemtible and what code is not? The
> rest of the i2c-algo-bit code should definitely _not_ be preemtible, as
> it is highly timing sensitive.

Code is preemptible when preempt_count() == 0 and interrupts are
enabled. spin_lock() implicitely disables preemption.
 
> > And as I said before nobody even noticed that the yield()
> > default implementation was changed to a NOOP by default in the
> > scheduler.
> 
> 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.
> 
> You say "NOOP by default", does this imply there is a way to change
> this?

There is a sysctl: sysctl_sched_compat_yield
 
> Was yield() turned into NOOP by design, or was it a bug?

By design. The semantics of yield and the fairness approach of CFS are
not really working well together. Also yield() for SCHED_OTHER is not
really specified.

Thanks,

	tglx
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