Re: [PATCH 0/7] preempt_count rework -v2

[Arjan van de Ven]

On 9/10/2013 6:56 AM, Ingo Molnar wrote:
> * 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.
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)


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