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

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Message-ID: <289e420d-a465-41c9-82ee-68ffbbfba0de@linux.intel.com>
Date: Fri, 17 Jan 2025 11:04:30 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: "H. Peter Anvin" <hpa@...or.com>, Ethan Zhao <etzhao@...look.com>,
 Xin Li <xin@...or.com>, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc: tglx@...utronix.de, dave.hansen@...ux.intel.com, x86@...nel.org,
 andrew.cooper3@...rix.com, mingo@...hat.com, bp@...en8.de
Subject: Re: [PATCH] x86/fred: Optimize the FRED entry by prioritizing
 high-probability event dispatching

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"