linux-kernel - Re: [PATCH 0/4] x86_64: Optimize percpu accesses

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Date:	Fri, 25 Jul 2008 17:27:11 -0700
From:	Mike Travis <travis@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Hugh Dickins <hugh@...itas.com>,
	Jack Steiner <steiner@....com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] x86_64: Optimize percpu accesses

Jeremy Fitzhardinge wrote:
> Mike Travis wrote:
>> This patchset provides the following:
>>
>>   * x86_64: Cleanup setup_percpu by fixing some minor potential
>>     problems as well as add some debugging aids.
>>
>>   * x86_64: Rebase per cpu variables to zero
>>
>>     Rebase per cpu variables to zero in preparation for the following
>>     patch to fold the pda into the per cpu area.
>>
>>   * x86_64: Fold pda into per cpu area
>>
>>     Declare the pda as a per cpu variable. This will allow the per cpu
>>     variables to be accessible on the x86_64 using %gs as the base of
>>     the percpu areas for each cpu:
>>
>>     %gs:per_cpu_xxxx
>>
>>   * x86_64: Reference zero-based percpu variables offset from gs
>>
>>     Actually implement the above operation for __get_cpu_var() and
>>     __put_cpu_var().  Since this is now a single instruction, we
>>     can remove the non-preemptible versions of x86_read_percpu()
>>     and x86_write_percpu().
>>   
> 
> No, I think you've misunderstood these calls.
> 
> get_cpu_var(x) evaluates to an lvalue of this cpu's 'x'.  It disables
> preemption, in the same manner as get_cpu().
> 
> put_cpu_var(x) does nothing more than re-enable preemption, to pair with
> get_cpu_var().
> 
> __get_cpu_var(x) is the same as get_cpu_var, but it assumes that
> preemption is already disabled.  There is no __put_cpu_var().
> 
> The important point is that an expression like "__get_cpu_var(x) = foo"
> does not evaluate to a single instruction, and is not preempt or
> interrupt -atomic.  That's the reason x86_X_percpu() exist, since
> they're a single instruction in an asm.  However, with %gs: based
> addressing they can be easily unified.
> 
>    J

Yes, you're right, I wrote that quickly without really reading it back.
My point is that now that x86_read_percpu() and x86_write_percpu() do
evaluate to a single instruction (by definition atomic), then it doesn't
need to be surrounded by the preempt_disable()/preempt_enable() calls.

It appears as if I'm implying that's the case for get/put_cpu_var().

Thanks,
Mike


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