linux-kernel - Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

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Message-ID: <1292003346.13513.30.camel@laptop>
Date:	Fri, 10 Dec 2010 18:49:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Venkatesh Pallipadi <venki@...gle.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Mikael Pettersson <mikpe@...uu.se>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	John Stultz <johnstul@...ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

On Fri, 2010-12-10 at 18:18 +0100, Eric Dumazet wrote:
> Le vendredi 10 décembre 2010 à 14:47 +0100, Peter Zijlstra a écrit :

> Also irq_time_write_begin() and irq_time_write_end() could be faster
> (called for current cpu)
> 
> static inline void irq_time_write_begin(void)
> {
> 	__this_cpu_inc(irq_time_seq.sequence);
> 	smp_wmb();
> }
>  
> static inline void irq_time_write_end(void)
> {
> 	smp_wmb();
> 	__this_cpu_inc(irq_time_seq.sequence);
> }

Yeah, but that kinda defeats the purpose of having it implemented in
seqlock.h. Ideally we'd teach gcc about these long pointers and have
something like: 

  write_seqcount_begin(&this_cpu_read(irq_time_seq));

do the right thing.
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