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SubjectRe: [PATCH 0/7] preempt_count rework -v2
On 9/10/2013 6:56 AM, Ingo Molnar wrote:
>
> * Ingo Molnar <mingo@kernel.org> 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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