Re: [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation

Steven Rostedt
2008-01-30T13:53:50Z

On Wed, 30 Jan 2008, Jan Kiszka wrote:

> Steven Rostedt wrote:
>
> > --- linux-mcount.git.orig/arch/x86/kernel/entry_32.S 2008-01-29 16:59:15.000000000 -0500
> > +++ linux-mcount.git/arch/x86/kernel/entry_32.S 2008-01-29 17:26:18.000000000 -0500
> > @@ -75,6 +75,31 @@ DF_MASK = 0x00000400
> > NT_MASK = 0x00004000
> > VM_MASK = 0x00020000
> >
> > +#ifdef CONFIG_MCOUNT
> > +.globl mcount
> > +mcount:
> > + /* unlikely(mcount_enabled) */
> > + cmpl $0, mcount_enabled
> > + jnz trace
> > + ret
>
> (and the corresponding 64-bit version)
>
> Is the impact of this change on the (already expensive) mcount_enabled
> case negligible? I worried about use cases where we want to gain some
> (relative) worst-case numbers via these instrumentations.

The goal here was to limit the instruction cache hit that we take when
mcount_enabled = 0.
>
> In my personal priority scheme, CONFIG_MCOUNT=y && !mcount_enabled comes
> after mcount_enabled.

well, actually, I disagree. I only set mcount_enabled=1 when I'm about to
test something. You're right that we want the impact of the test least
affected, but when we have mcount_enabled=1 we usually also have a
function that's attached and in that case this change is negligible. But
on the normal case where mcount_enabled=0, this change may have a bigger
impact.

Remember CONFIG_MCOUNT=y && mcount_enabled=0 is (15% overhead)
CONFIG_MCOUNT=y && mcount_enabled=1 dummy func (49% overhead)
CONFIG_MCOUNT=y && mcount_enabled=1 trace func (500% overhead)

The trace func is the one that will be most likely used when analyzing. It
gives hackbench a 500% overhead, so I'm expecting this change to be
negligible in that case. But after I find what's wrong, I like to rebuild
the kernel without rebooting so I like to have mcount_enabled=0 have the
smallest impact ;-)

I'll put back the original code and run some new numbers.

-- Steve