linux-kernel - Re: [PATCH 0/3] perfcounter: callchain symbol resolving and fixes

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Date:	Wed, 1 Jul 2009 18:33:25 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 0/3] perfcounter: callchain symbol resolving and fixes

On Wed, Jul 01, 2009 at 10:18:14AM +0200, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > Hi,
> > 
> > This patchset provides the symbol resolving for callchains.
> > Example:
> > 
> > perf report -s sym -c
> > 
> > 5.40%  [k] __d_lookup
> >              3.60%
> >                 __d_lookup
> >                 perf_callchain
> >                 perf_counter_overflow
> >                 intel_pmu_handle_irq
> >                 perf_counter_nmi_handler
> >                 notifier_call_chain
> >                 atomic_notifier_call_chain
> >                 notify_die
> >                 do_nmi
> >                 nmi
> >                 do_lookup
> >                 __link_path_walk
> >                 path_walk
> >                 do_path_lookup
> >                 user_path_at
> >                 vfs_fstatat
> >                 vfs_lstat
> >                 sys_newlstat
> >                 system_call_fastpath
> >                 __lxstat
> >                 0x406fb1
> 
> nice!
> 
> > Sorry about the third patch, it's a kind of all-in-one monolithic 
> > thing which gathers various fixes. I should have granulate it...
> 
> No problem, it's good enough - it's all about the same topic.
> 
> > 
> > Still in my plans:
> > 
> > - profit we have a tree to display a better graph hierarchy
> > - let the user provide a limit for hit percentage, depth, number of
> >   backtraces, etc...
> > - better output
> > - colors
> > 
> > And another one:
> > 
> > - remove the perfcounter internal nmi call frame (ie: every nmi frame)
> >   so that we drop this header from each callchain:
> > 
> >                 perf_callchain
> >                 perf_counter_overflow
> >                 intel_pmu_handle_irq
> >                 perf_counter_nmi_handler
> >                 notifier_call_chain
> >                 atomic_notifier_call_chain
> >                 notify_die
> >                 do_nmi
> >                 nmi
> 
> Sounds good. I suspect this latter one is the most important one 
> because right now the backtrace output screen real estate is 
> dominated by the repetitive nmi entries, making it hard to interpret 
> the result 'at a glance'.
> 
> I think we should skip those NMI entries right in the kernel - that 
> will also make call-chain event records quite a bit smaller, by 
> about 72 bytes per call-chain record.
> 
> We can do the skipping by using this backtrace-generator callback in 
> arch/x86/kernel/cpu/perf_counter.c:
> 
>  static int backtrace_stack(void *data, char *name)
>  {
>          /* Process all stacks: */
>          return 0;
>  }
> 
> The 'name' parameter passed in signals the type of stack frame we 
> are processing. If you look into arch/x86/kernel/dumpstack_64.c, it 
> can be one of these strings:
> 
>         static char ids[][8] = {
>                 [DEBUG_STACK - 1] = "#DB",
>                 [NMI_STACK - 1] = "NMI",
>                 [DOUBLEFAULT_STACK - 1] = "#DF",
>                 [STACKFAULT_STACK - 1] = "#SS",
>                 [MCE_STACK - 1] = "#MC",
> 
> A quick check to see whether this concept works would be expose the 
> ids array and do:
> 
>  static int PER_CPU(int, is_nmi_frame);
> 
>  static int backtrace_stack(void *data, char *name)
>  {
> 	if (name == x86_stack_ids[NMI_STACK-1])


IIRC, gcc manages to factorize the string table in the elf
format right?
So that a simple == should indeed work here.

Because if you look at dumpstack_64.c, the calls to ->stack()
use plain const string for some of them:

ops->stack(data, "IRQ")

But "NMI" is always passed by its real address in the ids so
that should work without problem here.

(I just feared about using strcmp is such a fastpath).



> 		per_cpu(is_nmi_frame, raw_processor_id()) = 1;
> 	else
> 		per_cpu(is_nmi_frame, raw_processor_id()) = 0;
> 
>         /* Process all stacks: */
>         return 0;
>  }
> 
> and to add something like this to backtrace_address():
> 
> 	if (per_cpu(is_nmi_frame, raw_processor_id())
> 		return;
> 
> 	Ingo


Heh, looks like I'll almost only have to copy-paste this mail :)

Another solution would be to handle an IGNORE return value
from dump_trace() instead of always terminate the trace when
->stack() < 0

Would it be useful for other kind of uses?
For now I just asssume ignoring a stack is not a known pattern
so I'll just implement your solution.

Thanks.

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