Messages in this thread | | | Date | Wed, 30 Jan 2008 08:53:24 -0500 (EST) | From | Steven Rostedt <> | Subject | Re: [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation |
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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
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