linux-kernel - Re: [RFC][PATCH 0/5] preempt

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Date:	Wed, 14 Aug 2013 09:14:34 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Mike Galbraith <bitbucket@...ine.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC][PATCH 0/5] preempt_count rework

On 08/14/2013 09:06 AM, Peter Zijlstra wrote:
> On Wed, Aug 14, 2013 at 05:39:11PM +0200, Mike Galbraith wrote:
>> On Wed, 2013-08-14 at 06:47 -0700, H. Peter Anvin wrote:
>>
>>> On x86, you never want to take the address of a percpu variable if you
>>> can avoid it, as you end up generating code like:
>>>
>>> 	movq %fs:0,%rax
>>> 	subl $1,(%rax)
>>
>> Hmmm..
>>
>> #define cpu_rq(cpu)             (&per_cpu(runqueues, (cpu)))
>> #define this_rq()               (&__get_cpu_var(runqueues))
>>
>> ffffffff81438c7f:       48 c7 c3 80 11 01 00    mov    $0x11180,%rbx
>>         /*
>>          * this_rq must be evaluated again because prev may have moved
>>          * CPUs since it called schedule(), thus the 'rq' on its stack
>>          * frame will be invalid.
>>          */
>>         finish_task_switch(this_rq(), prev);
>> ffffffff81438c86:       e8 25 b4 c0 ff          callq  ffffffff810440b0 <finish_task_switch>
>>                  * The context switch have flipped the stack from under us
>>                  * and restored the local variables which were saved when
>>                  * this task called schedule() in the past. prev == current
>>                  * is still correct, but it can be moved to another cpu/rq.
>>                  */
>>                 cpu = smp_processor_id();
>> ffffffff81438c8b:       65 8b 04 25 b8 c5 00    mov    %gs:0xc5b8,%eax
>> ffffffff81438c92:       00
>>                 rq = cpu_rq(cpu);
>> ffffffff81438c93:       48 98                   cltq
>> ffffffff81438c95:       48 03 1c c5 00 f3 bb    add    -0x7e440d00(,%rax,8),%rbx
>>
>> ..so could the rq = cpu_rq(cpu) sequence be improved cycle expenditure
>> wise by squirreling rq pointer away in a percpu this_rq, and replacing
>> cpu_rq(cpu) above with a __this_cpu_read(this_rq) version of this_rq()?
> 
> Well, this_rq() should already get you that. The above code sucks for
> using cpu_rq() when we know cpu == smp_processor_id().
> 

Even so, this_rq() uses __get_cpu_var() and takes its address, which
turns into a sequence like:

	leaq __percpu_runqueues(%rip),%rax
	addq %gs:this_cpu_off,%rax

... which is better than the above but still more heavyweight than it
would be if the pointer was itself a percpu variable.

	-hpa

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