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

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Date:	Fri, 12 Nov 2010 09:40:21 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Jamie Iles <jamie@...ieiles.com>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [Questions] How to run 'perf top' on ARM to profile kernel
 functions with modules loaded

2010/11/11 Peter Zijlstra <a.p.zijlstra@...llo.nl>:
> On Thu, 2010-11-11 at 12:21 +0000, Jamie Iles wrote:
>> 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@...il.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.
>
> Hrm,.. depending on how we load things this might be easy or hard to
> fix. If we load the module symbols after having loaded the kernel
> symbols we can replace ~0ULL with the address before the next symbol.

If possible, could you post a patch so that I can verify if the issue is caused
by ~0ULL?

> If however we load modules first we're in a bind and should look at
> changing the load order to make the first suggestion work.
>


thanks,
-- 
Lei Ming
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