* Ingo Molnar <mingo@xxxxxxxxxx> 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: 00
>
> and 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.
Thanks,
Ingo
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