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

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Date:	Wed, 14 Aug 2013 17:39:11 +0200
From:	Mike Galbraith <bitbucket@...ine.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 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()?

-Mike

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