On Mon, Dec 02, 2024 at 12:53:56PM -0800, Namhyung Kim wrote:

Hi Peter and Ingo,

On Tue, Nov 26, 2024 at 05:13:45PM -0500, Vince Weaver wrote:

quoted

On Sat, 23 Nov 2024, Mikołaj Kołek wrote:

quoted

findings. In the MMAP layout section of the page, you can find this
sentence:
Before Linux 2.6.39, there is a bug that means you must allocate
an mmap ring buffer when sampling even if you do not plan to
access it.
Unless I'm somehow misunderstanding it, this statement does not seem
to be well worded, or alternatively this bug does not seem to be
fixed.

That text was probably written by me.

I tried looking at the 2.6.39 code, my perf_tests, and also PAPI which was 
where the problem was probably noticed but I can't find a firm reference 
for how the issue was fixed.

If I recall, the problem was if you were trying to create a sampling event 
without mmap (say you want to get a signal every 100,000 retired 
instructions, but you don't actually want any sample data).  I think 
before 2.6.39 if you tried setting that up you'd get some sort of error 
(an EINVAL?) when trying to start(?) the event.

It is possible this wasn't fixed.  I tried to be pretty good 
about putting relevant git commits as comments in the manpage but there 
doesn't seem to be one for that part of the text.  I'm guessing it was 
PeterZ doing the work on this so maybe he remembers.

Do you remember what was the issue exactly on sampling events w/o mmap?

I can barely remember last release :/ But looking at the code, I don't
see a reason that wouldn't work today.

Only the overflow handler should care about there being a buffer, and
that just NOPs out if there isn't -- expensively, if this is a common
thing, this could perhaps be optimized a little.


---

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4c6f6c286b2d..190b5c3cec10 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c

@@ -8117,6 +8117,8 @@ __perf_event_output(struct perf_event *event,
 
 	/* protect the callchain buffers */
 	rcu_read_lock();
+	if (unlikely(!event->rb))
+		goto exit;
 
 	perf_prepare_sample(data, event, regs);
 	perf_prepare_header(&header, data, event, regs);