Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Lameter <cl@linux.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        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>
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Date: Fri, 10 Dec 2010 19:46:28 +0100
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On Fri, 2010-12-10 at 12:39 -0600, Christoph Lameter wrote:
> On Fri, 10 Dec 2010, Eric Dumazet wrote:
> 
> > > 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.
> >
> > gcc wont be able to do this yet (%fs/%gs selectors)
> 
> The kernel can do that using the __percpu annotation.

That's not true:

# define __percpu

Its a complete NOP.

> > But we can provide this_cpu_write_seqcount_{begin|end}()
> 
> No we cannot do hat. this_cpu ops are for per cpu data and not for locking
> values shared between processors. We have a mechanism for passing per cpu
> pointers with a corresponding annotation.

-enoparse, its not locking anything, is a per-cpu sequence count.


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