[Questions] How to run 'perf top' on ARM to profile kernel functions with modules loaded

[Jamie Iles]

On Thu, Nov 11, 2010 at 01:06:16PM +0100, Peter Zijlstra wrote:
> On Thu, 2010-11-11 at 09:17 +0000, Jamie Iles wrote:
> > On Thu, Nov 11, 2010 at 03:52:07PM +0800, Ming Lei wrote:
> > > 2010/11/11 Ming Lei <tom.leiming at gmail.com>:
> > > > Hi,
> > > >
> > > > 'perf top' can't display any functions when modules are loaded on ARM.
> > > 
> > > Sorry, should be can't display any kernel functions when modules are loaded
> > > on ARM.
> > I've noticed this myself and I spent a bit of time looking into it last week 
> > but didn't get too far. I did wonder if it was something to do with the map 
> > fixups after loading the modules. If you look at the raw events after a perf 
> > record with 'perf report -D' then the last module has a size that takes it to 
> > 0xFFFFFFFF which overlaps with the rest of the kernel symbols.
> > 
> > On x86 it looks like the modules are loaded after the kernel image in VM 
> > unlike ARM.
> 
> Order shouldn't matter, but that overlap is very likely what kills it.
> That module size is buggy.
I'm guessing that its in util/symbol.c:__map_groups__fixup_end():

	for (nd = rb_next(prevnd); nd; nd = rb_next(nd)) {
                prev = curr;
                curr = rb_entry(nd, struct map, rb_node);
                prev->end = curr->start - 1;
        }

        /*
         * We still haven't the actual symbols, so guess the
         * last map final address.
         */
        curr->end = ~0UL;

I've noticed that if I change the symbol_conf.{use_modules,try_vmlinux_path} 
then I can get perf events for kernel symbols from /proc/kallsyms but nothing 
for the modules.

Jamie