Re: [PATCH 4/4] perf core: Add backward attribute to perf event

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

On 2016/3/29 22:04, Peter Zijlstra wrote:
> On Mon, Mar 28, 2016 at 06:41:32AM +0000, Wang Nan wrote:
>
> Could you maybe write a perf/tests thingy for this so that _some_
> userspace exists that exercises this new code?
>
>
>>   int perf_output_begin(struct perf_output_handle *handle,
>>   		      struct perf_event *event, unsigned int size)
>>   {
>> +	if (unlikely(is_write_backward(event)))
>> +		return __perf_output_begin(handle, event, size, true);
>>   	return __perf_output_begin(handle, event, size, false);
>>   }
> Would something like:
>
> int perf_output_begin(...)
> {
> 	if (unlikely(is_write_backward(event))
> 		return perf_output_begin_backward(...);
> 	return perf_output_begin_forward(...);
> }
>
> make sense; I'm not sure how much is still using this, but it seems
> somewhat excessive to inline two copies of that thing into a single
> function.

perf_output_begin is useful:

$ grep perf_output_begin ./kernel -r
./kernel/events/ring_buffer.c:     * See perf_output_begin().
./kernel/events/ring_buffer.c:int perf_output_begin(struct 
perf_output_handle *handle,
./kernel/events/ring_buffer.c:     * perf_output_begin() only checks 
rb->paused, therefore
./kernel/events/core.c:    if (perf_output_begin(&handle, event, 
header.size))
./kernel/events/core.c:    ret = perf_output_begin(&handle, event, 
read_event.header.size);
./kernel/events/core.c:    ret = perf_output_begin(&handle, event,
./kernel/events/core.c:    ret = perf_output_begin(&handle, event,
./kernel/events/core.c:    ret = perf_output_begin(&handle, event,
./kernel/events/core.c:    ret = perf_output_begin(&handle, event, 
rec.header.size);
./kernel/events/core.c:    ret = perf_output_begin(&handle, event,
./kernel/events/core.c:    ret = perf_output_begin(&handle, event, 
se->event_id.header.size);
./kernel/events/core.c:    ret = perf_output_begin(&handle, event,
./kernel/events/core.c:    ret = perf_output_begin(&handle, event, 
rec.header.size);

Events like PERF_RECORD_MMAP2 uses this function, so we still need to 
consider its overhead.

So I will use your first suggestion.

Thank you.