Date:	Wed, 1 Jul 2009 19:14:20 +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 06:33:25PM +0200, Frederic Weisbecker wrote:
> 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.


Actually, the strings mapped to stack ids are
not const pointers so they won't go the string table I guess.
Anyway "NMI" is never passed as a plain string so it should work
fine.



> 
> 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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