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 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
