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From: Ingo Molnar <mingo@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Hansen <dave@sr71.net>, Brian Gerst <brgerst@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Chang S . Bae" <chang.seok.bae@intel.com>
Subject: Re: question about switch_fpu_prepare/switch_fpu_finish
Date: Tue, 22 Apr 2025 22:11:48 +0200	[thread overview]
Message-ID: <aAf4BNLmpMV7I_My@gmail.com> (raw)
In-Reply-To: <20250422170059.GB1676@redhat.com>


* Oleg Nesterov <oleg@redhat.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

  reply	other threads:[~2025-04-22 20:11 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-04-09 21:11 [PATCH -v5 0/8] sched: Make task_struct::thread constant size, x86/fpu: Remove thread::fpu Ingo Molnar
2025-04-09 21:11 ` [PATCH 1/8] x86/fpu: Introduce the x86_task_fpu() helper method Ingo Molnar
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-09 21:11 ` [PATCH 2/8] x86/fpu: Convert task_struct::thread.fpu accesses to use x86_task_fpu() Ingo Molnar
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-09 21:11 ` [PATCH 3/8] x86/fpu: Make task_struct::thread constant size Ingo Molnar
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-09 21:11 ` [PATCH 4/8] x86/fpu: Remove the thread::fpu pointer Ingo Molnar
2025-04-10  7:39   ` Peter Zijlstra
2025-04-10 10:10     ` Ingo Molnar
2025-04-10 10:30       ` Peter Zijlstra
2025-04-10 10:54         ` [PATCH] x86/fpu: Clarify FPU context cacheline alignment Ingo Molnar
2025-04-14  7:34           ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-10 10:51       ` [PATCH 4/8] x86/fpu: Remove the thread::fpu pointer Ingo Molnar
2025-04-10 14:04       ` Oleg Nesterov
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-09 21:11 ` [PATCH 5/8] x86/fpu: Push 'fpu' pointer calculation into the fpu__drop() call Ingo Molnar
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-09 21:11 ` [PATCH 6/8] x86/fpu: Make sure x86_task_fpu() doesn't get called for PF_KTHREAD|PF_USER_WORKER tasks during exit Ingo Molnar
2025-04-11 15:22   ` Chang S. Bae
2025-04-12  8:37     ` Ingo Molnar
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-09 21:11 ` [PATCH 7/8] x86/fpu: Remove init_task FPU state dependencies, add debugging warning for PF_KTHREAD tasks Ingo Molnar
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-09 21:11 ` [PATCH 8/8] x86/fpu: Use 'fpstate' variable names consistently Ingo Molnar
2025-04-14  7:34   ` [tip: x86/merge] " tip-bot2 for Ingo Molnar
2025-04-22 16:11 ` [PATCH -v5 0/8] sched: Make task_struct::thread constant size, x86/fpu: Remove thread::fpu Oleg Nesterov
2025-04-22 20:09   ` Ingo Molnar
2025-04-22 17:01 ` question about switch_fpu_prepare/switch_fpu_finish Oleg Nesterov
2025-04-22 20:11   ` Ingo Molnar [this message]
2025-05-03 14:38 ` [PATCH tip/x86/fpu 1/6] x86/fpu: simplify the switch_fpu_prepare() + switch_fpu_finish() logic Oleg Nesterov
2025-05-04  8:54   ` [tip: x86/fpu] x86/fpu: Simplify " tip-bot2 for Oleg Nesterov
2025-05-03 14:38 ` [PATCH tip/x86/fpu 2/6] x86/fpu: kill x86_init_fpu Oleg Nesterov
2025-05-04  8:54   ` [tip: x86/fpu] x86/fpu: Remove x86_init_fpu tip-bot2 for Oleg Nesterov
2025-05-03 14:38 ` [PATCH tip/x86/fpu 3/6] x86/fpu: kill DEFINE_EVENT(x86_fpu, x86_fpu_copy_src) Oleg Nesterov
2025-05-04  8:54   ` [tip: x86/fpu] x86/fpu: Remove " tip-bot2 for Oleg Nesterov
2025-05-03 14:38 ` [PATCH tip/x86/fpu 4/6] x86/fpu: arch_dup_task_struct: always use memcpy_and_pad() Oleg Nesterov
2025-05-04  8:54   ` [tip: x86/fpu] x86/fpu: Always use memcpy_and_pad() in arch_dup_task_struct() tip-bot2 for Oleg Nesterov
2025-05-03 14:38 ` [PATCH tip/x86/fpu 5/6] x86/fpu: fpu__drop: check TIF_NEED_FPU_LOAD instead of PF_KTHREAD|PF_USER_WORKER Oleg Nesterov
2025-05-04  8:54   ` [tip: x86/fpu] x86/fpu: Check TIF_NEED_FPU_LOAD instead of PF_KTHREAD|PF_USER_WORKER in fpu__drop() tip-bot2 for Oleg Nesterov
2025-05-03 14:39 ` [PATCH tip/x86/fpu 6/6] x86/fpu: shift fpregs_assert_state_consistent() from arch_exit_work() to its caller Oleg Nesterov
2025-05-04  8:36   ` Ingo Molnar
2025-05-04  8:54   ` [tip: x86/fpu] x86/fpu: Shift " tip-bot2 for Oleg Nesterov

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