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

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Message-ID: <d90975a0-6b01-4a2e-92c2-2af2326e1299@zytor.com>
Date: Thu, 16 Jan 2025 17:21:16 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Ethan Zhao <etzhao@...look.com>, Xin Li <xin@...or.com>,
        Ethan Zhao <haifeng.zhao@...ux.intel.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 1/16/25 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)))

	-hpa