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: > > 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? 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);
