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

Wangnan (F)
2016-03-30T02:29:29Z

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.