linux-kernel - Re: [PATCH -v5 0/8] sched: Make task_struct::thread constant size, x86/fpu: Remove thread::fpu

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Message-ID: <20250422161108.GA1676@redhat.com>
Date: Tue, 22 Apr 2025 18:11:08 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: 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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Chang S . Bae" <chang.seok.bae@...el.com>
Subject: Re: [PATCH -v5 0/8] sched: Make task_struct::thread constant size,
 x86/fpu: Remove thread::fpu

Ingo, sorry for delay.

So just in case, the whole series looks good to me. I am going to send a
couple of minor cleanups on top of it, but let me ask first if I missed
something or not.

- x86_init_fpu is not really used after 4/8, it can be killed

- DEFINE_EVENT(x86_fpu, x86_fpu_copy_src) can be killed after 7/8

- arch_dup_task_struct() still does

	/* init_task is not dynamically sized (incomplete FPU state) */
	if (unlikely(src == &init_task))
		memcpy_and_pad(dst, arch_task_struct_size, src, sizeof(init_task), 0);
	else
		memcpy(dst, src, arch_task_struct_size);

  and I don't understand why do we need to check src == &init_task. It seems
  that we can always do

	memcpy_and_pad(dst, arch_task_struct_size, src, sizeof(struct task_struct), 0);

  or even just

	memcpy(dst, src, sizeof(struct task_struct));

  fpu_clone() will initialize the "dst_fpu" memory correctly.

- fpu__drop() does

	/* PF_KTHREAD tasks do not use the FPU context area: */
	if (tsk->flags & (PF_KTHREAD | PF_USER_WORKER))
		return;

  and this is correct. But perhaps

	if (test_tsk_thread_flag(tsk, TIF_NEED_FPU_LOAD))
		return;

  makes more sense? PF_KTHREAD's should never clear TIF_NEED_FPU_LOAD,
  and this way we can avoid the unnecessary "fwait" if, say, the exiting
  task does context_switch() at least once on its way to exit_thread().

- Finally, with or without these changes, it seems that the
  switch_fpu_prepare() + switch_fpu_finish() logic can be simplified,
  I'll write another email.