Messages in this thread | | | Subject | Re: [PATCH 2/3] x86: Remove cmpxchg from i386 NMI nesting code | From | Steven Rostedt <> | Date | Fri, 08 Jun 2012 13:36:15 -0400 |
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On Fri, 2012-06-08 at 10:28 -0700, H. Peter Anvin wrote: > On 06/08/2012 09:41 AM, Steven Rostedt wrote: > > >> > >> #define nmi_nesting_postprocess() \ > >> do { \ > >> - if (cmpxchg(&__get_cpu_var(nmi_state), \ > >> - NMI_EXECUTING, NMI_NOT_RUNNING) != NMI_EXECUTING) \ > >> + if (!local_dec_and_test(&__get_cpu_var(nmi_state))) \ > > > > Now this is where I think the patch helps. I'm almost certain that > > local_dec_and_test is faster than a cmpxchg by many cycles. Especially > > on i386. > > > > On i386 it's infinite, but again, I don't think the code will ever be > exercised on i386. I'm much more concerned about performance on current > processors.
I'm not worried about non i386 here. Some context missing from the patch is that this code is surrounded by:
#ifdef CONFIG_X86_32
[...]
#else
The '#else' part does something completely different, as this work is done in the assembly handler on x86_64.
And yes, this code is executed on i386 (well when I boot my x86_64 into an i386 kernel it does).
> > But yes, local_dec_and_test should at least not be more expensive. Even > better, use this_cpu_dec_return().
Oh! That would make this much better! I'll update the patch set.
Thanks!
-- Steve
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