Re: [RFC][PATCH 0/5] preempt_count rework

Mike Galbraith
2013-08-14T15:39:30Z

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