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From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Suresh Siddha <sbsiddha@gmail.com>,
	Bean Anderson <bean@azulsystems.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/5] x86, fpu: don't drop_fpu() in __restore_xstate_sig() if use_eager_fpu()
Date: Mon, 25 Aug 2014 19:39:30 +0200	[thread overview]
Message-ID: <20140825173930.GA10376@redhat.com> (raw)
In-Reply-To: <CA+55aFw_-_h9Rj8F1H-nRm2-x8wMZ=f0ee122UXCVyJ_seuk2w@mail.gmail.com>

On 08/25, Linus Torvalds wrote:
>
> On Mon, Aug 25, 2014 at 10:09 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > I'd like to change it only because this code is the main source of the
> > nasty special case, used_math() and/or __thread_has_fpu(current) can be
> > false even if use_eager_fpu().
>
> Well, if you think it is correct (apart from missing preemption), then
> I disagree *violently* with your "clean it up by restoring things
> twice" model.

OK.

> The signal handling overhead of floating point restore is not small,
> and it's not theoretical.

Again, this only if 32bit && use_eager_fpu(), and use_eager_fpu() adds
this overhead to every context switch.

But you convinced me anyway. If this hack doesn't look "obviously safe
and correct", lets forget it.

I'll try to play with copy_from_user_in_atomic(), if nothing else just
to complete the discussion and see how the code can look in this case.

Thanks!

Oleg.


  reply	other threads:[~2014-08-25 17:42 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-08-22 17:11 [PATCH 0/1] x86, fpu: shift drop_init_fpu() from save_xstate_sig() to handle_signal() Oleg Nesterov
2014-08-22 17:12 ` [PATCH 1/1] " Oleg Nesterov
2014-08-24 19:47 ` [PATCH 0/5] x86, fpu: make use_eager_fpu() more eager Oleg Nesterov
2014-08-24 19:47   ` [PATCH 1/5] x86, fpu: change sanitize_restored_xstate() and convert_to_fxsr() to accept thread_xstate Oleg Nesterov
2014-08-24 19:47   ` [PATCH 2/5] x86, fpu: don't drop_fpu() in __restore_xstate_sig() if use_eager_fpu() Oleg Nesterov
2014-08-24 20:05     ` Linus Torvalds
2014-08-25 14:26       ` Oleg Nesterov
2014-08-25 14:41       ` Oleg Nesterov
2014-08-25 16:27         ` Linus Torvalds
2014-08-25 17:09           ` Oleg Nesterov
2014-08-25 17:26             ` Linus Torvalds
2014-08-25 17:39               ` Oleg Nesterov [this message]
2014-08-27 17:02                 ` Oleg Nesterov
2014-08-24 19:47   ` [PATCH 3/5] x86, fpu: don't drop_fpu() in exit_thread() " Oleg Nesterov
2014-08-24 19:47   ` [PATCH 4/5] x86, fpu: shift init_fpu() from eager_fpu_init() to eager_fpu_init_bp() Oleg Nesterov
2014-08-24 19:47   ` [PATCH 5/5] x86, fpu: sanitize the usage of use_eager_fpu() in switch_fpu_prepare() Oleg Nesterov
2014-08-25 18:08 ` [PATCH] x86, fpu: __restore_xstate_sig()->math_state_restore() needs preempt_disable() Oleg Nesterov
2014-09-02  5:01   ` Suresh Siddha

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