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Date: Thu, 18 May 2017 18:44:25 +0900
From: Namhyung Kim <namhyung@kernel.org>
To: Milian Wolff <milian.wolff@kdab.com>
Cc: Linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        David Ahern <dsahern@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Yao Jin <yao.jin@linux.intel.com>, kernel-team@lge.com
Subject: Re: [PATCH v3] perf report: fix off-by-one for non-activation frames
Message-ID: <20170518094425.GD4885@sejong>
References: <20170518083735.14126-1-milian.wolff@kdab.com>
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Hi Milian,

On Thu, May 18, 2017 at 10:37:35AM +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;
>   }
> ~~~~~~~~~~~~~~~
> 
> 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
> ~~~~~~~~~~~~~~~
> 
> Similarly it works for signal frames:
> 
> ~~~~~~~~~~~~~~~
> #include <stdio.h>
> #include <stdlib.h>
> #include <signal.h>
> 
> #define __noinline  __attribute__((noinline))
> 
> __noinline void bar(void)
> {
>   volatile long cnt = 0;
> 
>   for (cnt = 0; cnt < 100000000; cnt++);
> }
> 
> __noinline void foo(void)
> {
>   bar();
> }
> 
> void sig_handler(int sig)
> {
>   foo();
> }
> 
> int main(void)
> {
>   signal(SIGUSR1, sig_handler);
>   raise(SIGUSR1);
> 
>   foo();
>   return 0;
> }
> ~~~~~~~~~~~~~~~~
> 
> Before, the report wrongly points to `signal.c:29` after raise():
> 
> ~~~~~~~~~~~~~~~~
> $ perf report --stdio --no-children -g srcline -s srcline
> ...
>    100.00%  signal.c:11
>             |
>             ---bar signal.c:11
>                |
>                |--50.49%--main signal.c:29
>                |          __libc_start_main
>                |          _start
>                |
>                 --49.51%--0x33a8f
>                           raise .:0
>                           main signal.c:29
>                           __libc_start_main
>                           _start
> ~~~~~~~~~~~~~~~~
> 
> With this patch in, the issue is fixed and we instead get:
> 
> ~~~~~~~~~~~~~~~~
>    100.00%  signal   signal            [.] bar
>             |
>             ---bar signal.c:11
>                |
>                |--50.49%--main signal.c:29
>                |          __libc_start_main
>                |          _start
>                |
>                 --49.51%--0x33a8f
>                           raise .:0
>                           main signal.c:27
>                           __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@redhat.com>
> Cc: David Ahern <dsahern@gmail.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Yao Jin <yao.jin@linux.intel.com>
> Signed-off-by: Milian Wolff <milian.wolff@kdab.com>

Acked-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


> ---
>  tools/perf/util/unwind-libdw.c           |  6 +++++-
>  tools/perf/util/unwind-libunwind-local.c | 11 +++++++++++
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> v2:
> - use unw_is_signal_frame to also fix this issue for libunwind
> 
> v3:
> - only check current signal in libunwind case, which fixes the
>   example code provided by Namhyung. The example is added in the
>   commit message and I confirmed that libdw and libunwind now
>   both produce the correct results for this case
> 
> 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..84d553898e2a 100644
> --- a/tools/perf/util/unwind-libunwind-local.c
> +++ b/tools/perf/util/unwind-libunwind-local.c
> @@ -692,6 +692,17 @@ static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
>  
>  		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.
> +			 */
> +			if (unw_is_signal_frame(&c) <= 0)
> +				--ips[i];
> +
>  			++i;
>  		}
>  
> -- 
> 2.13.0
>