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Date: Tue, 16 May 2017 23:38:29 +0900
From: Namhyung Kim <namhyung@...nel.org>
To: Milian Wolff <milian.wolff@...b.com>
Cc: Linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
Arnaldo Carvalho de Melo <acme@...hat.com>,
David Ahern <dsahern@...il.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Yao Jin <yao.jin@...ux.intel.com>, kernel-team@....com
Subject: Re: [PATCH v2] perf report: fix off-by-one for non-activation frames
On Tue, May 16, 2017 at 10:59:51AM +0200, Milian Wolff wrote:
> As the documentation for dwfl_frame_pc says, frames that
> are no activation frames need to have their program counter
> decremented by one to properly find the function of the caller.
>
> This fixes many cases where perf report currently attributes
> the cost to the next line. I.e. I have code like this:
>
> ~~~~~~~~~~~~~~~
> #include <thread>
> #include <chrono>
>
> using namespace std;
>
> int main()
> {
> this_thread::sleep_for(chrono::milliseconds(1000));
> this_thread::sleep_for(chrono::milliseconds(100));
> this_thread::sleep_for(chrono::milliseconds(10));
>
> return 0;
> }
> ~~~~~~~~~~~~~~~
It'd be nice if the test program has a signal frame for verification.
>
> Now compile and record it:
>
> ~~~~~~~~~~~~~~~
> g++ -std=c++11 -g -O2 test.cpp
> echo 1 | sudo tee /proc/sys/kernel/sched_schedstats
> perf record \
> --event sched:sched_stat_sleep \
> --event sched:sched_process_exit \
> --event sched:sched_switch --call-graph=dwarf \
> --output perf.data.raw \
> ./a.out
> echo 0 | sudo tee /proc/sys/kernel/sched_schedstats
> perf inject --sched-stat --input perf.data.raw --output perf.data
> ~~~~~~~~~~~~~~~
>
> Before this patch, the report clearly shows the off-by-one issue.
> Most notably, the last sleep invocation is incorrectly attributed
> to the "return 0;" line:
>
> ~~~~~~~~~~~~~~~
> Overhead Source:Line
> ........ ...........
>
> 100.00% core.c:0
> |
> ---__schedule core.c:0
> schedule
> do_nanosleep hrtimer.c:0
> hrtimer_nanosleep
> sys_nanosleep
> entry_SYSCALL_64_fastpath .tmp_entry_64.o:0
> __nanosleep_nocancel .:0
> std::this_thread::sleep_for<long, std::ratio<1l, 1000l> > thread:323
> |
> |--90.08%--main test.cpp:9
> | __libc_start_main
> | _start
> |
> |--9.01%--main test.cpp:10
> | __libc_start_main
> | _start
> |
> --0.91%--main test.cpp:13
> __libc_start_main
> _start
> ~~~~~~~~~~~~~~~
>
> With this patch here applied, the issue is fixed. The report becomes
> much more usable:
>
> ~~~~~~~~~~~~~~~
> Overhead Source:Line
> ........ ...........
>
> 100.00% core.c:0
> |
> ---__schedule core.c:0
> schedule
> do_nanosleep hrtimer.c:0
> hrtimer_nanosleep
> sys_nanosleep
> entry_SYSCALL_64_fastpath .tmp_entry_64.o:0
> __nanosleep_nocancel .:0
> std::this_thread::sleep_for<long, std::ratio<1l, 1000l> > thread:323
> |
> |--90.08%--main test.cpp:8
> | __libc_start_main
> | _start
> |
> |--9.01%--main test.cpp:9
> | __libc_start_main
> | _start
> |
> --0.91%--main test.cpp:10
> __libc_start_main
> _start
> ~~~~~~~~~~~~~~~
>
> Note how this patch fixes this issue for both unwinding methods, i.e.
> both dwfl and libunwind. The former case is straight-forward thanks
> to dwfl_frame_pc. For libunwind, we replace the functionality via
> unw_is_signal_frame for any but the very first frame.
>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: David Ahern <dsahern@...il.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Yao Jin <yao.jin@...ux.intel.com>
> Signed-off-by: Milian Wolff <milian.wolff@...b.com>
> ---
> tools/perf/util/unwind-libdw.c | 6 +++++-
> tools/perf/util/unwind-libunwind-local.c | 14 ++++++++++++++
> 2 files changed, 19 insertions(+), 1 deletion(-)
>
> v2:
> - use unw_is_signal_frame to also fix this issue for libunwind
>
> diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c
> index f90e11a555b2..943a06291587 100644
> --- a/tools/perf/util/unwind-libdw.c
> +++ b/tools/perf/util/unwind-libdw.c
> @@ -168,12 +168,16 @@ frame_callback(Dwfl_Frame *state, void *arg)
> {
> struct unwind_info *ui = arg;
> Dwarf_Addr pc;
> + bool isactivation;
>
> - if (!dwfl_frame_pc(state, &pc, NULL)) {
> + if (!dwfl_frame_pc(state, &pc, &isactivation)) {
> pr_err("%s", dwfl_errmsg(-1));
> return DWARF_CB_ABORT;
> }
>
> + if (!isactivation)
> + --pc;
> +
> return entry(pc, ui) || !(--ui->max_stack) ?
> DWARF_CB_ABORT : DWARF_CB_OK;
> }
> diff --git a/tools/perf/util/unwind-libunwind-local.c b/tools/perf/util/unwind-libunwind-local.c
> index f8455bed6e65..30ab26375c80 100644
> --- a/tools/perf/util/unwind-libunwind-local.c
> +++ b/tools/perf/util/unwind-libunwind-local.c
> @@ -690,8 +690,22 @@ static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
> if (ret)
> display_error(ret);
>
> + bool previous_frame_was_signal = false;
> while (!ret && (unw_step(&c) > 0) && i < max_stack) {
> unw_get_reg(&c, UNW_REG_IP, &ips[i]);
> +
> + /*
> + * Decrement the IP for any non-activation frames.
> + * this is required to properly find the srcline
> + * for caller frames.
> + * See also the documentation for dwfl_frame_pc,
> + * which this code tries to replicate.
> + */
> + bool frame_is_signal = unw_is_signal_frame(&c) > 0;
> + if (!previous_frame_was_signal && !frame_is_signal)
> + --ips[i];
> + previous_frame_was_signal = frame_is_signal;
Does it need to check previous frame too?
Thanks,
Namhyung
> +
> ++i;
> }
>
> --
> 2.13.0
>
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