linux-kernel - Re: question about switch_fpu_prepare/switch_fpu

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Message-ID: <aAf4BNLmpMV7I_My@gmail.com>
Date: Tue, 22 Apr 2025 22:11:48 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave@...1.net>, Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Chang S . Bae" <chang.seok.bae@...el.com>
Subject: Re: question about switch_fpu_prepare/switch_fpu_finish


* Oleg Nesterov <oleg@...hat.com> wrote:

> I must have missed something, but I can't understand this logic, it
> seems unnecessarily complicated today.
> 
> 1. Now that switch_fpu_finish() doesn't load the FPU state, I think it
>    can be folded into switch_fpu_prepare().

Agreed.

> 2. But the main question is that I fail to understand why
>    __switch_to() -> switch_fpu_finish() uses the "next" task to set
>    TIF_NEED_FPU_LOAD.
> 
>    I think that set_tsk_thread_flag(prev_p, TIF_NEED_FPU_LOAD) makes
>    more sense.
> 
>    Just in case, note that fpu_clone() sets TIF_NEED_FPU_LOAD, so
>    we should not worry about the 1st __switch_to(next_p).
> 
> IOW, can you explain why the (untested) patch below could be wrong?

I think your patch should work.

> We can even remove the PF_KTHREAD check in switch_fpu_prepare(), kthreads
> should never clear TIF_NEED_FPU_LOAD...

Agreed.

Thanks,

	Ingo