Re: [patch] i386: use thread_info flags for debug regs and IO bitmaps

[Chuck Ebbert <76306.1226@compuserve.com>]

In-Reply-To: <Pine.LNX.4.64.0607081425430.3869@g5.osdl.org>

On Sat, 8 Jul 2006 14:26:53 -0700, Linus Torvalds wrote:
> 
> On Fri, 7 Jul 2006, Chuck Ebbert wrote:
> >
> > From: Stephane Eranian <eranian@hpl.hp.com>
> > 
> > Use thread info flags to track use of debug registers and IO bitmaps.
> >  
> >     - add TIF_DEBUG to track when debug registers are active
> >     - add TIF_IO_BITMAP to track when I/O bitmap is used
> >     - modify __switch_to() to use the new TIF flags
> 
> Can you explain what the advantages of this are?

Stephane's perfmon2 patch adds yet another special-case to the
switch_to() code, so Andi suggested this change.  It will allow
the perfmon2 patch to have no performance impact on normal
task-switching, since it will just use another flag.

After I saw a ~7% gain in task-switch performance, I like it now
even without perfmon2 in there.

> I don't see it. It's just creating new state to describe state that we 
> already had, and as far as I can tell, it's just a way to potentially have 
> more new bugs thanks to the new state getting out of sync with the old 
> one?

Well yeah, there is that.  But Andi and I both reviewed it and he's
already put the x86_64 version into his tree.  Testing in -mm should
show whether there are any problems.

-- 
Chuck
 "You can't read a newspaper if you can't read."  --George W. Bush