Re: [PATCH v8 2/8] perf evlist: Introduce aux evlist

["Wangnan (F)" <wangnan0@huawei.com>]

On 2016/6/21 4:36, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jun 20, 2016 at 10:47:19AM +0000, Wang Nan escreveu:
>> An auxiliary evlist is created by perf_evlist__new_aux() using an
>> existing evlist as its parent. An auxiliary evlist can have its own
>> 'struct perf_mmap', but can't have any other data. User should use its
>> parent instead when accessing other data.
>>
>> Auxiliary evlists are containers of 'struct perf_mmap'. It is introduced
>> to allow its parent evlist to map different events into separated mmaps.
>>
>> Following commits create an auxiliary evlist for overwritable
>> events, because overwritable events need a read only and backwards ring
>> buffer, which is different from normal events.
>>
>> To achieve this goal, this patch carefully changes 'evlist' to
>> 'evlist->parent' in all functions in the path of 'perf_evlist__mmap_ex',
>> except 'evlist->mmap' related operations, to make sure all evlist
>> modifications (like pollfd and event id hash tables) goes to original
>> evlist.
>>
>> A 'evlist->parent' pointer is added to 'struct perf_evlist' and points to
>> the evlist itself for normal evlists.
>>
>> Children of one evlist are linked into it so one can find all children
>> from its parent.
>> To avoid potential complexity, forbid creating aux evlist from another
>> aux evlist.
>>
>> Improve perf_evlist__munmap_filtered(), so when recording, if an event
>> is terminated, unmap mmaps, from parent and children.
>   
>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>> Cc: He Kuang <hekuang@huawei.com>
>> Cc: Jiri Olsa <jolsa@kernel.org>
>> Cc: Masami Hiramatsu <mhiramat@kernel.org>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: Zefan Li <lizefan@huawei.com>
>> Cc: pi3orama@163.com
>> ---
>>   tools/perf/util/evlist.c | 49 +++++++++++++++++++++++++++++++++++++-----------
>>   tools/perf/util/evlist.h | 12 ++++++++++++
>>   2 files changed, 50 insertions(+), 11 deletions(-)

[SNIP]

>> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
>> index 68cb136..5b50692 100644
>> --- a/tools/perf/util/evlist.h
>> +++ b/tools/perf/util/evlist.h
>> @@ -37,6 +37,10 @@ struct perf_mmap {
>>   
>>   struct perf_evlist {
>>   	struct list_head entries;
>> +	union {
>> +		struct list_head children;
>> +		struct list_head list;
> This is something new, right, i.e. having a list of evlists from a
> parent evlist, one where there can be at most one level, i.e. no
> grandchildrens...
>
> Is this strictly needed? I.e. the existing code calling
> perf_evlist__munmap_filtered() has just the parent pointer and thus, to
> not change it we need to have a way to go the children?
>
> Also, can you se a parent with more than one child?

If we further restrict aux evlist and parent list, allow only one aux
evlist for a parent, we can avoid the linked list. Simply linking parent
and child using pointers would be enough.

However, I plan to introduce multiple children for a parent. There are
two possible aux evlists:

   1. Trigger aux evlist: we can use BPF script to measure events, and 
do something
      (dumpping, printing, computing, reporting...) when it detect something
      (for example, FPS drop to 10 frames/sec). We can create a bpf-output
      event and put it in a separated aux evlist with buffering turned off,
      so once BPF script generate a event perf receives it immediately.

   2. Non-sampling aux evlist: we can use a dummy event to collect fork, 
comm
      and mmap{,2} events in a separated aux evlist. We can use this 
evlist to
      maintain system state during recording, so we can totally solve 
the problem
      describe in patch 8/8.

Thank you.