linux-kernel - Re: 2.6.24-rc6-mm1

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Message-Id: <200801022131.15482.nickpiggin@yahoo.com.au>
Date:	Wed, 2 Jan 2008 21:31:15 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	trem <tremyfr@...oo.fr>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.24-rc6-mm1

On Monday 31 December 2007 00:10, Ingo Molnar wrote:
> * Herbert Xu <herbert@...dor.apana.org.au> wrote:
> > > Ingo, it's not good that we have cond_resched() definitions
> > > conditionally duplicated in kernel.h - that's increasing the risk of
> > > bugs like this one.
> >
> > Actually, why do we even have cond_resched when real preemption is on?
> > It seems to be a waste of space and time.
>
> due to the BKL. cond_resched() in BKL code breaks up BKL latencies.
>
> i dont mind not doing that though - we should increase the pain for BKL
> users, so that subsystems finally get rid of it altogether.
> lock_kernel() use within the kernel is still rampant - there are still
> more than 400 callsites to lock_kernel().

It would be silly to potentially increase latency in some areas
for CONFIG_PREEMPT kernels, though.

Better may be to detect when there is CONFIG_PREEMPT and
CONFIG_PREEMPT_BKL, and ifdef away the cond_resched in that case
(or -- why do we even make CONFIG_PREEMPT_BKL an option? Are there
really workloads left where it causes throughput regressions?)
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