Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints

[Jason Baron <jbaron@redhat.com>]

On Tue, Oct 19, 2010 at 09:36:30PM -0400, Steven Rostedt wrote:
> 
> On Wed, 2010-10-20 at 00:17 +0200, Thomas Gleixner wrote:
> > On Tue, 19 Oct 2010, Thomas Gleixner wrote:
> > > On Tue, 19 Oct 2010, Jason Baron wrote:
> > > > > Now even worse, when you NOP out the jmpq then your tracepoint is
> > > > > still not enabled. Brilliant !
> > > > > 
> > > > 
> > > > The 'jmpq' in the enabled case is patched with a jmpq to the body of the
> > > > tracepoint itself.
> > > 
> > > Brilliant.
> > 
> > IOW, We now jump around the jump which jumps around the disabled code.
> > 
> 
> 
> Do you happen to have CONFIG_CC_OPTIMIZE_FOR_SIZE set? If so, then this
> is a known issue. We even originally had jump label enabled _only_ if
> CC_OPTIMIZE_FOR_SIZE was not set, but hpa NAK'd it.
> 
> http://lkml.org/lkml/2010/9/22/482
> 
> http://lkml.org/lkml/2010/9/20/488
> 
> http://lkml.org/lkml/2010/9/24/259
> 
> -- Steve

thanks Steve. I was about to say this. When CONFIG_CC_OPTIMIZE_FOR_SIZE
is not set we don't get the double 'jmp' and the tracepoint code is
moved out of line. It was mentioned that a number of distros ship with
CONFIG_CC_OPTIMIZE_FOR_SIZE not set, and as Steve mentioned my original
patch set was conditional on !CONFIG_CC_OPTIMIZE_FOR_SIZE.

using hot/cold labels gcc can fix the CONFIG_CC_OPTIMIZE_FOR_SIZE case,
but its a non-trivial amount of work for gcc. I was hoping that if jump
labels are included, we could make the gcc work happen.

thanks,

-Jason