Re: [PATCH] x86/fred: Optimize the FRED entry by prioritizing high-probability event dispatching

[Ethan Zhao <haifeng.zhao@linux.intel.com>]

在 2025/1/17 9:21, H. Peter Anvin 写道:
> On 1/16/25 16:37, Ethan Zhao wrote:
>>>
>>> hpa suggested to introduce "switch_likely" for this kind of 
>>> optimization
>>> on a switch statement, which is also easier to read.  I measured it 
>>> with
>>> a user space focus test, it does improve performance a lot. But 
>>> obviously there are still a lot of work to do.
>>
>> Find a way to instruct compiler to pick the right hot branch 
>> meanwhile make folks
>> reading happy... yup, a lot of work.
>>
>
> It's not that complicated, believe it or not.
>
> /*
>  * switch(v) biased for speed in the case v == l
>  *
>  * Note: gcc is quite sensitive to the exact form of this
>  * expression.
>  */
> #define switch_likely(v,l) \
>     switch((__typeof__(v))__builtin_expect((v),(l)))
>
I know we could play such trick for one branch, never think of there are 
2-3 branches
are expected among 7 branches. :) , --- external interrupts, syscall, 
page fault.

Thanks,
Ethan
>     -hpa
>
-- 
"firm, enduring, strong, and long-lived"