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

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Message-ID:
 <TYZPR03MB880148D071B32806DBB1ACFFD1E52@TYZPR03MB8801.apcprd03.prod.outlook.com>
Date: Sat, 18 Jan 2025 11:29:36 +0800
From: Ethan Zhao <etzhao@...look.com>
To: "H. Peter Anvin" <hpa@...or.com>,
 Ethan Zhao <haifeng.zhao@...ux.intel.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

On 1/18/2025 12:24 AM, H. Peter Anvin wrote:
>>
>> In short, seems that __builtin_expect not work with switch(), at 
>> least for
>>   gcc version 8.5.0 20210514(RHEL).
>>
>
> For forward-facing optimizations, please don't use an ancient version 
> of gcc as the benchmark.

Even there is a latest Gcc built-in feature could work for this case, it 
is highly unlikely that Linus would adopt such trick into upstream 
kernel (only works for specific ver compiler). the same resultto those 
downstream vendors/LTS kernels. thus, making an optimization with latest 
only Gcc would construct an impractical benchmark-only performance 
barrier. As to the __builtin_expect(), my understanding, it was designed 
to only work for if(bool value) {
}
else if(bool value) {
} The value of the condition expression returned by __builtin_expect() 
is a bool const. while switch(variable) expects a variable. so it is 
normal for Gcc that it doesn't work with it.

If I got something wrong, please let me know.

Thanks,
Ethan

>
>     -hpa
>