Re: perf_event_open.2: mmap ring buffer requirement for receiving overflow notifications

[Namhyung Kim <namhyung@kernel.org>]

Hi Peter and Ingo,

On Tue, Nov 26, 2024 at 05:13:45PM -0500, Vince Weaver wrote:
> On Sat, 23 Nov 2024, Mikołaj Kołek wrote:
> 
> > 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?

Thanks,
Namhyung