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

From: Wangnan (F)
Date: Tue Mar 29 2016 - 22:29:32 EST




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.