linux-kernel - Re: [RFC][PATCH] x86, fpu: dynamically allocate 'struct fpu'

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Message-ID: <20150717084527.GC16130@gmail.com>
Date:	Fri, 17 Jul 2015 10:45:27 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] x86, fpu: dynamically allocate 'struct fpu'


* Dave Hansen <dave@...1.net> wrote:

> >>  #endif /* _ASM_X86_FPU_H */
> >> diff -puN arch/x86/kernel/process.c~dynamically-allocate-struct-fpu arch/x86/kernel/process.c
> >> --- a/arch/x86/kernel/process.c~dynamically-allocate-struct-fpu 2015-07-16 10:50:42.360571875 -0700
> >> +++ b/arch/x86/kernel/process.c 2015-07-16 12:00:59.204808551 -0700
> >> @@ -81,7 +81,7 @@ EXPORT_SYMBOL_GPL(idle_notifier_unregist
> >>   */
> >>  int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> >>  {
> >> -       *dst = *src;
> >> +       memcpy(dst, src, arch_task_struct_size());
> > 
> > This is actually vaguely performance-critical, which makes me thing
> > that using some kind of inline or other real way (config macro, ifdef,
> > etc) to detect whether there's an arch override would be better than a
> > weak function.
> 
> Fair enough.  I'll send out another version in a bit if there are no more 
> comments.

Beyond making it a build time switch for other architectures, I'd also suggest 
introducing a __read_mostly task_struct_size variable on x86, so that we can 
write:

	memcpy(dst, src, task_struct_size);

Thanks,

	Ingo
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