linux-kernel - RE: [PATCH] perf probe: Adjust dso->long

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Message-ID: <50399556C9727B4D88A595C8584AAB3752631912@GSjpTKYDCembx32.service.hitachi.net>
Date:	Thu, 26 Nov 2015 01:10:32 +0000
From:	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
To:	"'Wang Nan'" <wangnan0@...wei.com>,
	"acme@...nel.org" <acme@...nel.org>
CC:	"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Arnaldo Carvalho de Melo" <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	"Zefan Li" <lizefan@...wei.com>,
	"pi3orama@....com" <pi3orama@....com>
Subject: RE: [PATCH] perf probe: Adjust dso->long_name for offline module

From: Wang Nan [mailto:wangnan0@...wei.com]
>
>If libelf unable to open debuginfo for an offline module but the ko has
>symtab, something unexpected may happen.
>
> # rm -rf ~/.debug/
> # mv /usr/lib64/elfutils/libebl_x86_64.so{,.bak}

Please do give more possible usecase. removing libebl is crazy,
and broken environment.

If you'd like to use perf probe without debuginfo, make NO_DWARF=1 or
strip the target binary.

> # ./perf probe -m /home/wangnan/kmodule/mymodule.ko my_func
> [mymodule] with build id 326ab42550ef3d24944f53c817533728367effeb not found, continuing without symbols
> Failed to find symbol my_func in /home/wangnan/kmodule/mymodule.ko
>   Error: Failed to add events
> # ./perf buildid-cache -a ./mymodule.ko
> # ./perf probe -m /home/wangnan/kmodule/mymodule.ko my_func
> Added new event:
>   probe:my_func        (on my_func in /home/wangnan/kmodule/mymodule.ko)
>
> You can now use it in all perf tools, such as:
>
>	 perf record -e probe:my_func -aR sleep 1
>
>In the above example, probe fails if it isn't in buildid-cache. However,
>user would expect it success in both case because perf is able to find
>probe points actually.
>
>The problem is because perf won't utilize module's full path if it
>failed to open debuginfo. In
> convert_to_probe_trace_events ->
> find_probe_trace_events_from_map ->
> get_target_map ->
> kernel_get_module_map ->
> machine__findnew_module_map ->
> map_groups__find_by_name
>
>map_groups__find_by_name() is able to find the map of that module, but
>this information is found from /proc/modules before it knows the real
>path of the offline module. Therefore, the map->dso->long_name is
>set to something like '[mymodule]', which prevents dso__load() find
>the real path of the module file.

Hmm, if so, it should be fixed in map or machine, not in probe-event.c.

Thanks,