Re: [PATCH 1/2] coredump: flush the fpu exit state for proper multi-threaded core dump

From: Suresh Siddha
Date: Tue May 08 2012 - 19:15:39 EST

On Mon, 2012-05-07 at 13:09 -0700, Suresh Siddha wrote:
On Mon, 2012-05-07 at 12:15 -0700, Linus Torvalds wrote:
> > Ugh, I hate this one.
> >
> > We're making the exit path more expensive for almost no gain. The FPU
> > state is dead in 99.9% of all cases.
> >
> > Why isn't this a core-dump-only case?
> >
> Today we do this unlazy_fpu() (which is what prepare_to_copy does)
> already as part of the first schedule() in the exit path. I am just
> making it explicit by calling prepare_to_copy() before exit_mm() which
> synchronizes all the threads before the first thread goes with the core
> dump.
> I don't think I am adding any more cost to the existing path, unless I
> am missing something.

My quick test confirmed my understanding. For example, mainline is
doing fpu_save_init() some 1770 times when a tsk is in the TASK_DEAD state,
during boot of a two socket system.

So based on your suggestion, made the first patch as core-dump only case and
added the third patch which clears the fpu state during exit.


Suresh Siddha (3):
coredump: flush the fpu exit state for proper multi-threaded core
x86, xsave: remove thread_has_fpu() bug check in
x86, fpu: clear the fpu state during thread exit

arch/x86/kernel/process.c | 19 +++++++++++++------
arch/x86/kernel/xsave.c | 2 --
kernel/exit.c | 5 +++++
3 files changed, 18 insertions(+), 8 deletions(-)


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