linux-kernel - Re: [PATCH 3/3] x86/fpu: Remove init_task FPU state dependencies, add debugging warning

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Message-ID: <CAHk-=wi4773Ls82kqVbPmM1deghS2NXkHNCCzWPc270kcByXPA@mail.gmail.com>
Date: Wed, 5 Jun 2024 09:08:07 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, 
	Andy Lutomirski <luto@...capital.net>, Andrew Morton <akpm@...ux-foundation.org>, 
	Dave Hansen <dave@...1.net>, Peter Zijlstra <peterz@...radead.org>, Borislav Petkov <bp@...en8.de>, 
	"H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH 3/3] x86/fpu: Remove init_task FPU state dependencies, add
 debugging warning

On Wed, 5 Jun 2024 at 07:19, Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 06/05, Ingo Molnar wrote:
> >
> > But the init task isn't supposed to be using the FPU in any case,
>
> Afaics, the same is true for any PF_KTHREAD/USER_WORKER thread?

I don't think so. We have various users of kernel_fpu_begin()/end()
that are very much about things like crypto and RAID xor memory copies
etc that will be used by kernel worker threads.

In fact, as far as I know, we'll use the FPU in the init_task too
thanks to irq_fpu_usable(). Look up the  nft_pipapo_avx2_lookup()
function.

Maybe other patches removed that, I didn't check the context, but the
"init_task doesn't use FPU" doesn't actually sound true to me.

                Linus