Re: updating COMPAT_LINUX for linux 2.6.x support (take 2)

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Fri, Jun 18, 2010 at 04:09:30AM +1000, matthew green wrote:
> 
> i ran a bunch of measurements for sparc64 when i changed GENERIC to be
> MP by default (it's necessary for now, until we parse the numa-like
> memory maps and avoid memory not available without depending on the CPU
> it is physically attached to.)
> 
> i couldn't observe any difference in a abunch of real world and micro-
> benchmark cases, so i never even bothered with patching away code that
> "isn't necessary" (though i've written code to do said patching.)

In the x86 case, what's patched away on uniprocessors is the prefix
that causes locked bus transactions.  The effect is definitely measurable!

FBT is a little different.  FBT patches *in* the code that actually
implements the trace points when you turn them on.  Since the trace
points may include conditional branches and other code features which
could have an impact on the execution of _other_ code as a side effect,
it's nice to know they aren't there at all unless you turn them on; the
cpu cannot (mis-)speculatively execute code that is not there. :-)

Thor