Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Venkatesh Pallipadi <venki@google.com>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Mikael Pettersson <mikpe@it.uu.se>, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        John Stultz <johnstul@us.ibm.com>,
        Christoph Lameter <cl@linux-foundation.org>
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Date: Fri, 10 Dec 2010 18:49:06 +0100
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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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