linux-kernel - Re: [PATCH 0/3] Optimize code generation during context switching

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Message-ID: <20251025122659.GA2352457@noisy.programming.kicks-ass.net>
Date: Sat, 25 Oct 2025 14:26:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Xie Yuanbin <qq570070308@...il.com>
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Subject: Re: [PATCH 0/3] Optimize code generation during context switching

On Sat, Oct 25, 2025 at 02:26:25AM +0800, Xie Yuanbin wrote:
> The purpose of this series of patches is to optimize the performance of
> context switching. It does not change the code logic, but only modifies
> the inline attributes of some functions.
> 
> The original reason for writing this patch is that, when debugging a
> schedule performance problem, I discovered that the finish_task_switch
> function was not inlined, even in the O2 level optimization. This may
> affect performance for the following reasons:

Not sure what compiler you're running, but it is on the one random
compile I just checked.

> 1. It is in the context switching code, and is called frequently.
> 2. Because of the modern CPU mitigations for vulnerabilities, inside
> switch_mm, the instruction pipeline and cache may be cleared, and the
> branch and cache miss may increase. finish_task_switch is right after
> that, so this may cause greater performance degradation.

That patch really is one of the ugliest things I've seen in a while; and
you have no performance numbers included or any other justification for
any of this ugly.

> 3. The __schedule function has __sched attribute, which makes it be
> placed in the ".sched.text" section, while finish_task_switch does not,
> which causes their distance to be very far in binary, aggravating the
> above performance degradation.

How? If it doesn't get inlined it will be a direct call, in which case
the prefetcher should have no trouble.