Re: [PATCH 1/3] perf tools: Allow vmlinux to fallback to kallsyms on NO_LIBELF=1

From: Namhyung Kim
Date: Fri Nov 07 2014 - 09:57:52 EST


Hi Peter,

2014-11-07 (ê), 09:27 +0100, Peter Zijlstra:
> On Fri, Nov 07, 2014 at 02:20:04PM +0900, Namhyung Kim wrote:
> > When libelf is not used, perf cannot not show symbol names since it
> > doesn't access the ELF symbol table. But kernel is different as it
> > can fallback to kallsyms.
> >
> > It worked w/o libelf, but recent change to use vmlinux for kernel
> > symbols break it.
> >
> > With this change, it now can show kernel symbols again:
> >
> > # Overhead Command Shared Object Symbol
> > # ........ ....... ................. ........................
> > #
> > 34.51% swapper [kernel.kallsyms] [k] intel_idle
> > 12.54% perf [kernel.kallsyms] [k] generic_exec_single
> > 10.11% swapper [kernel.kallsyms] [k] int_sqrt
> > 9.83% swapper [kernel.kallsyms] [k] hrtimer_interrupt
> > 7.25% emacs [kernel.kallsyms] [k] __switch_to
> > 7.06% sleep [kernel.kallsyms] [k] find_next_iomem_res
> > 7.02% swapper [kernel.kallsyms] [k] run_timer_softirq
> > 2.55% swapper [kernel.kallsyms] [k] _raw_spin_unlock_irq
> > 1.90% swapper [kernel.kallsyms] [k] lapic_next_deadline
> > 1.75% swapper [kernel.kallsyms] [k] native_sched_clock
> > 1.49% swapper [kernel.kallsyms] [k] __schedule
> > 1.20% swapper [kernel.kallsyms] [k] read_tsc
> > 1.10% sleep [kernel.kallsyms] [k] current_kernel_time
> > 0.99% swapper [kernel.kallsyms] [k] pick_next_task_rt
> > 0.36% swapper [kernel.kallsyms] [k] group_sched_in
> > 0.24% swapper [kernel.kallsyms] [k] x86_pmu_commit_txn
> > 0.06% swapper [kernel.kallsyms] [k] intel_pmu_enable_all
> > 0.03% perf [kernel.kallsyms] [k] intel_pmu_enable_all
> >
> > Reported-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > ---
> > tools/perf/util/symbol-minimal.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/tools/perf/util/symbol-minimal.c b/tools/perf/util/symbol-minimal.c
> > index c9541fea9514..226cf41ed7e6 100644
> > --- a/tools/perf/util/symbol-minimal.c
> > +++ b/tools/perf/util/symbol-minimal.c
> > @@ -335,6 +335,9 @@ int dso__load_sym(struct dso *dso, struct map *map __maybe_unused,
> > unsigned char *build_id[BUILD_ID_SIZE];
> > int ret;
> >
> > + if (dso->kernel)
> > + return 0; /* always use kallsyms */
> > +
> > ret = fd__is_64_bit(ss->fd);
> > if (ret >= 0)
> > dso->is_64_bit = ret;
>
> Why does this live in the minimal implementation; should we not always
> discard ELF files with 0 symbols?
>
> Suppose I have a vmlinux but removed all symbols from it; I want it to
> fall back to kallsyms too.

I'm not sure I understood what you said correctly. With this change,
dso__load_kernel_sym() always ends up calling dso__load_kallsyms() since
dso__load_vmlinux() will always return 0;

So I think you'll fall back to kallsyms even though you have a vmlinux
with symbol. This makes dso__load_sym() in the patch 2/3 simpler IMHO.

Thanks,
Namhyung


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