On 9/10/2013 6:56 AM, Ingo Molnar wrote:
* Ingo Molnar <[email protected]> wrote:So what we do in kick_process() is: preempt_disable(); cpu = task_cpu(p); if ((cpu != smp_processor_id()) && task_curr(p)) smp_send_reschedule(cpu); preempt_enable(); The preempt_disable() looks sweet:ffffffff8106f3f1: 65 ff 04 25 e0 b7 00 incl %gs:0xb7e0 ffffffff8106f3f8: 00and the '*' you marked is the preempt_enable() portion, which, with your new code, looks like this: #define preempt_check_resched() \ do { \ if (unlikely(!*preempt_count_ptr())) \ preempt_schedule(); \ } while (0) Which GCC translates to:* ffffffff8106f42a: 65 ff 0c 25 e0 b7 00 decl %gs:0xb7e0 ffffffff8106f431: 00 * ffffffff8106f432: 0f 94 c0 sete %al * ffffffff8106f435: 84 c0 test %al,%al * ffffffff8106f437: 75 02 jne ffffffff8106f43b <kick_process+0x4b>Correction, so this comes from the new x86-specific optimization: +static __always_inline bool __preempt_count_dec_and_test(void) +{ + unsigned char c; + + asm ("decl " __percpu_arg(0) "; sete %1" + : "+m" (__preempt_count), "=qm" (c)); + + return c != 0; +} And that's where the sete and test originates from. Couldn't it be improved by merging the preempt_schedule() call into a new primitive, keeping the call in the regular flow, or using section tricks to move it out of line? The scheduling case is a slowpath in most cases.
also.. yuck on using "dec" "dec" sucks, please use "sub foo ,1" instead (dec sucks because of its broken flags behavior; it creates basically a bubble in the pipeline) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
