linux-kernel - [PATCH v2 0/3] More i387 state save/restore work

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Date:	Mon, 20 Feb 2012 11:46:32 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
cc:	x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 0/3] More i387 state save/restore work

This is a slightly updated patch-series that does the same thing my
previous one did, just fixing a few special cases.

In particular, I noticed that the 'fpu_counter' situation was still
rather confused in the last patch-series, and instead of having patch#1
fix up a part of it as part of re-organizing the FP state restore code,
this just fixes the fpu_counter confusion separately.  Especially since
I wanted to clear the fpu_counter at fork time for new processes, just
to make sure tht we never try to lazily switch to stale FPU state
(re-used task struct pointer and all that). 

So patch#1 is that fpu_counter work.

The two other patches are basically the same as last time, except with 
slightly updated commit logs and obviously updated for the fpu_counter 
fixes (it is, for example, wrong to update the counter at FPU preload 
time: if we do the lazy restore, there won't be any explicit preload, but 
the fpu_counter should still update - so that affected the preloading 
thing).

I've also done more testing of the series, although some of that shows
funny effects: it's actually faster on my machine (and in my specialized
tests) to switch between two processes where *one* uses the FPU with
these lazy restore patches than it is to switch between two non-FPU
users. 

However, that seems to be due to some funny cache interaction: the
profiles clearly show that '__switch_to()' itself is much faster if it
doesn't have to work with any FPU state at all.  But probably because of
some random cache replacement detail, I then get more switches with the
more expensive __switch_to when I only save the FP state (without ever
restoring it).

Regardless, I'm pretty happy with this series, and I think I'll commit
and push out at least the two first patches just because they fix real
(albeit not very important) bugs.  I feel like I'm going to do the third
one too for 3.3, but I'm not sure yet.  And comments welcome.

                Linus

Linus Torvalds (3):
  i387: fix up some fpu_counter confusion
  i387: use 'restore_fpu_checking()' directly in task switching code
  i387: support lazy restore of FPU state

 arch/x86/include/asm/i387.h      |   53 +++++++++++++++++++++++++++----------
 arch/x86/include/asm/processor.h |    3 +-
 arch/x86/kernel/cpu/common.c     |    2 +
 arch/x86/kernel/process_32.c     |    3 +-
 arch/x86/kernel/process_64.c     |    3 +-
 arch/x86/kernel/traps.c          |   40 +++++-----------------------
 6 files changed, 54 insertions(+), 50 deletions(-)

-- 
1.7.9.188.g12766.dirty

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