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Date: Tue, 11 Jun 2024 14:42:13 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Nathan Chancellor <nathan@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
	Andy Lutomirski <luto@amacapital.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave@sr71.net>, Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Uros Bizjak <ubizjak@gmail.com>,
	Nathan Chancellor <nathan@kernel.org>
Subject: Re: [PATCH 3/9] x86/fpu: Make task_struct::thread constant size
Message-ID: <20240611124145.GA26798@redhat.com>
References: <20240608073134.264210-1-mingo@kernel.org>
 <20240608073134.264210-4-mingo@kernel.org>
 <20240610211350.GA1613053@thelio-3990X>
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I don't think this can explain the problem reported by Nathan, but.

On 06/08, Ingo Molnar wrote:
>
> +static struct fpu x86_init_fpu __read_mostly;
> +
>  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;

OK,

> +	per_cpu(fpu_fpregs_owner_ctx, this_cpu) = &x86_init_fpu;
> +	x86_init_fpu.last_cpu = this_cpu;

Why? I think it should do

	x86_init_fpu.last_cpu = -1;
	set_thread_flag(TIF_NEED_FPU_LOAD);

And the next patch should kill x86_init_fpu altogether, but keep
TIF_NEED_FPU_LOAD. It should be never cleared if PF_KTHREAD.

Oleg.