Re: [PATCH perf/core v3 3/3] [BUGFIX] perf probe: Show usage even if the last event is skipped

From: Masami Hiramatsu
Date: Wed Jun 17 2015 - 01:31:26 EST

On 2015/06/16 23:46, Arnaldo Carvalho de Melo wrote:
> Em Tue, Jun 16, 2015 at 08:50:57PM +0900, Masami Hiramatsu escreveu:
>> When the last part of converted events are blacklisted or out-of-text,
>> those are skipped and perf probe doesn't show usage examples.
>> This fixes it to show the example even if the last part of event list
>> is skipped.
>> E.g. without this patch, events are added, but suddenly end;
> End what? Stop being added?

"End without the last message", I meant.

> I.e. not all eligible events are added? From
> your description the problem seems to be that that last message: "You
> can now use it..." is not presented, but here, without this patch, it
> is:

I see, actually, this happens only if the skipped symbols (
vfs_caches_init_early or vfs_caches_init) are placed at the end of the
matched symbol list. On Ubuntu 15.04 kernel, it doesn't have
vfs_load_quota_inode etc. and the vfs_caches_init is the last part of
the matched list. Since it is hard to reproduce, I've added a Note on
the end of description :)


>> Note that this can be reproduced ONLY IF the vfs_caches_init*
>> is the last part of matched symbol list. I've checked this happens
>> on "3.19.0-generic #18-Ubuntu" kernel binary.


To reproduce this bug, you need to find a good symbol matching pattern
which (1) matches both of valid function in .text and invalid function
in .inittext (2) invalid one must be on the end of matched function list.

I fortunately hit such pattern and found this bug, but it depends on
the kernel binary.


> I.e. the only problem I found was this:
> [root@zoo ~]# time perf probe -l > /dev/null
> real 0m15.408s
> user 0m14.892s
> sys 0m0.534s
> [root@zoo ~]#
> [root@zoo ~]# perf stat perf probe -l > /dev/null
> Performance counter stats for 'perf probe -l':
> 15256.588897 task-clock (msec) # 1.001 CPUs utilized
> 116 context-switches # 0.008 K/sec
> 4 cpu-migrations # 0.000 K/sec
> 230,720 page-faults # 0.015 M/sec
> 47,830,405,530 cycles # 3.135 GHz
> 43,974,134,505 stalled-cycles-frontend # 91.94% frontend cycles idle
> <not supported> stalled-cycles-backend
> 11,540,587,038 instructions # 0.24 insns per cycle
> # 3.81 stalled cycles per insn
> 2,807,769,592 branches # 184.037 M/sec
> 20,087,075 branch-misses # 0.72% of all branches
> 15.240796324 seconds time elapsed
> [root@zoo ~]#
> Can you check why it takes so long and check the need for this patch?

It is because perf probe -l is not optimized to show a lot of probes yet.
It initializes and loads debuginfo for each probe. I guess we can reuse
debuginfo among them. let me try...

Thank you,

Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
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