linux-kernel - Re: [PATCH 3/9] x86/fpu: Make task

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Message-ID: <20240612203052.GC23973@redhat.com>
Date: Wed, 12 Jun 2024 22:30:52 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Nathan Chancellor <nathan@...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>, Brian Gerst <brgerst@...il.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH 3/9] x86/fpu: Make task_struct::thread constant size

Damn, sorry for the additional spam, but if I was not clear...

On 06/12, Oleg Nesterov wrote:
>
> The patch below seems to fix the problem.

Of course I am not trying to propose this change. This is just
the debugging patch. Which can hopefully explain the problem.

> Again, the changes in fpu__init_system_early_generic() are not
> strictly needed to fix it, but I believe make sense anyway.

...

>  static void __init fpu__init_system_early_generic(void)
>  {
> -	int this_cpu = smp_processor_id();
> -
>  	fpstate_reset(&x86_init_fpu);
>  	current->thread.fpu = &x86_init_fpu;
> -	per_cpu(fpu_fpregs_owner_ctx, this_cpu) = &x86_init_fpu;
> -	x86_init_fpu.last_cpu = this_cpu;
> +	set_thread_flag(TIF_NEED_FPU_LOAD);
> +	x86_init_fpu.last_cpu = -1;

Yes, in particular set_thread_flag(TIF_NEED_FPU_LOAD). And x86_init_fpu
should die after the next patch.

Oleg.