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Message-ID: <20090701171419.GA6264@nowhere>
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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