Re: [PATCH] perf util: Use target->per_thread and target->system_wide flags

["Jin, Yao" <yao.jin@linux.intel.com>]

On 1/23/2018 5:10 AM, Mathieu Poirier wrote:
> On 22 January 2018 at 15:15, Jin Yao <yao.jin@linux.intel.com> wrote:
>> Mathieu Poirier reports issue in commit ("73c0ca1eee3d perf thread_map:
>> Enumerate all threads from /proc") that it has negative impact on
>> 'perf record --per-thread'. It has the effect of creating a kernel event
>> for each thread in the system for 'perf record --per-thread'.
>>
>> Mathieu Poirier's patch ("perf util: Do not reuse target->per_thread flag")
>> can fix this issue by creating a new target->all_threads flag.
>>
>> This patch is based on Mathieu Poirier's patch but it doesn't use a new
>> target->all_threads flag. This patch just uses 'target->per_thread &&
>> target->system_wide' as a condition to check for all threads case.
>>
>> Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
>> ---
>>   tools/perf/util/evlist.c     | 2 +-
>>   tools/perf/util/thread_map.c | 4 ++--
>>   tools/perf/util/thread_map.h | 2 +-
>>   3 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
>> index 120efd8..9dff74a 100644
>> --- a/tools/perf/util/evlist.c
>> +++ b/tools/perf/util/evlist.c
>> @@ -1106,7 +1106,7 @@ int perf_evlist__create_maps(struct perf_evlist *evlist, struct target *target)
>>          struct thread_map *threads;
>>
>>          threads = thread_map__new_str(target->pid, target->tid, target->uid,
>> -                                     target->per_thread);
>> +                                     target->per_thread && target->system_wide);
> 
> At first glance I thought your solution would do the trick but perf
> record does use target->system_wide when the '-a' switch is used.
> Moreover specifying the '-a' switch doesn't prevent the '--per-thread'
> option from being used as well, making both target->perf_thread and
> target_system_wide equal to true (and that is not good).
> 
> Although not a fan of adding more to struct target, the advantage of
> having target->all_threads is that we are guaranteed that it isn't
> used anywhere else.
> 
> Let me know what you think,
> Mathieu
> 

If we specify both '-a' and '--per-thread' to perf record, perf record 
will override'--per-thread'. So now target->per_thread = false, and 
target->system_wide = true.

If we specify '--per-thread' only to perf record, target->per_thread = 
true, and target->system_wide = false.

So whatever for any case, target->per_thread && target->system_wide is 
false.

Since the parameter is false, in thread_map__new_str(), it will not 
execute the thread_map__new_all_cpus(). So that will not change perf 
record previous behavior.

In perf stat, it allows the case that target->per_thread and 
target->system_wide are all true. That means we want to collect 
system-wide per-thread metrics.

That's my current thinking.

Thanks
Jin Yao