From: Linus Torvalds <torvalds@linux-foundation.org>
To: Suresh Siddha <sbsiddha@gmail.com>
Cc: Nate Eldredge <nate@thatsmathematics.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
"the arch/x86 maintainers" <x86@kernel.org>,
stable <stable@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Maarten Baert <maarten-baert@hotmail.com>,
Jan Kara <jack@suse.cz>, George Spelvin <linux@horizon.com>,
Pekka Riikonen <priikone@iki.fi>
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt flag
Date: Sat, 1 Feb 2014 11:27:54 -0800 [thread overview]
Message-ID: <CA+55aFz-=9L=K2O-yB5f9R-w40PzXJ5xEXcinnUdUF1QHO7KMQ@mail.gmail.com> (raw)
In-Reply-To: <CALmL7E9LRZCPH5EuS_Jm-iJzpKnWj4OsXmS44VYDpLoDKPfVxA@mail.gmail.com>
On Thu, Jan 30, 2014 at 11:33 PM, Suresh Siddha <sbsiddha@gmail.com> wrote:
>
> a. delayed dynamic allocation of FPU state area was not a good idea
> (from me). Given most of the future cases will be anyway using eager
> FPU (because of processor features like xsaveopt etc, applications
> implicitly using FPU because of optimizations in commonly used
> libraries etc), we should probably go back to allocation of FPU state
> area during thread creation for everyone (including non-eager cases).
Yes, I suspect that will help some, and probably fix this particular bug.
That said, regardless of the allocation issue, I do think that it's
stupid for kernel_fpu_{begin,end} to save the math state if
"used_math" was not set. So I do think__kernel_fpu_end() as-s is
buggy and stupid. So I do think we should *either* say
(a) "we don't want to restore at all, because once the kernel starts
using math, it might do so a lot, and saving/restoring is a bad idea":
void __kernel_fpu_end(void)
{
stts();
}
*or*
(b) make the use_eager_fpu() case check tsk_used_math() (in which
case we had better already have an allocation!)
void __kernel_fpu_end(void)
{
if (use_eager_fpu()) {
struct task_struct *me = current;
if (tsk_used_math(me) && likely(!restore_fpu_checking(me)))
return;
}
stts();
}
Quite frankly, I'd almost lean towards (a). Comments? Does anybody
have any loads where the kernel does a lot of fpu stuff (ie network
encryption using the hw engines or something)? I'd really like to hear
if it makes a difference..
Linus
next prev parent reply other threads:[~2014-02-01 19:27 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-30 22:01 [PATCH] Make math_state_restore() save and restore the interrupt flag Nate Eldredge
2014-01-30 22:24 ` Linus Torvalds
2014-01-31 7:33 ` Suresh Siddha
2014-02-01 19:27 ` Linus Torvalds [this message]
2014-02-01 19:35 ` H. Peter Anvin
2014-02-01 19:46 ` Linus Torvalds
2014-02-01 20:00 ` H. Peter Anvin
2014-02-01 20:16 ` Linus Torvalds
2014-02-01 20:16 ` H. Peter Anvin
2014-02-01 21:17 ` George Spelvin
2014-02-01 21:36 ` H. Peter Anvin
2014-02-01 23:40 ` H. Peter Anvin
2014-02-02 0:17 ` Linus Torvalds
2014-02-02 1:19 ` George Spelvin
2014-02-02 1:25 ` H. Peter Anvin
2014-02-02 8:45 ` Pekka Riikonen
2014-02-02 1:06 ` Suresh Siddha
2014-02-02 1:26 ` H. Peter Anvin
2014-02-02 1:35 ` Suresh Siddha
2014-02-02 1:38 ` Linus Torvalds
2014-02-02 1:47 ` Suresh Siddha
2014-02-02 1:51 ` Linus Torvalds
2014-02-02 1:57 ` H. Peter Anvin
2014-02-02 2:05 ` Linus Torvalds
2014-02-02 2:12 ` H. Peter Anvin
2014-02-02 1:59 ` Suresh Siddha
2014-02-02 1:43 ` H. Peter Anvin
2014-02-02 1:47 ` Linus Torvalds
2014-02-02 7:19 ` Suresh Siddha
2014-02-02 19:15 ` Linus Torvalds
2014-02-03 6:56 ` Suresh Siddha
2014-02-03 18:20 ` Linus Torvalds
2014-02-04 6:03 ` Suresh Siddha
2014-02-06 5:26 ` Nate Eldredge
2014-02-06 5:34 ` George Spelvin
2014-02-13 15:45 ` Maarten Baert
2014-02-13 20:00 ` George Spelvin
2014-03-11 19:36 ` [tip:x86/urgent] x86, fpu: Check tsk_used_math() in kernel_fpu_end() for eager FPU tip-bot for Suresh Siddha
2014-02-27 23:44 ` [PATCH] Make math_state_restore() save and restore the interrupt flag H. Peter Anvin
2014-03-07 23:18 ` H. Peter Anvin
2014-03-08 6:18 ` Suresh Siddha
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