Hi Namhyung, On Tue, Aug 22, 2023 at 09:26:05PM -0700, Namhyung Kim wrote: [...] > > > > +Why ring buffers are mapped with above two different modes? Here the > > > > +write direction matters. The forward writing starts to save data from > > > > +the beginning of the ring buffer and wrap around when overflow, which is > > > > +used with the read-write mode in the normal ring buffer. When the > > > > +consumer doesn't keep up with the producer, it would lose some data, the > > > > +kernel keeps how many records it lost and generates the > > > > +``PERF_RECORD_LOST`` records in the next time when it finds a space in the > > > > +ring buffer. > > > > > > Thanks for the update. It's unclear to me if all 4 combination of > > > (rw, ro) x (fwd, bwd) are possible (yes!). The rw mode and back- > > > ward is also possible but just not used for perf tool. > > > > I can add a matrix for the combinations: > > > > The combination is supported in perf tool: > > > > ---+------------+----------- > > | Forward | Backward > > ---+------------+----------- > > rw | Yes | No > > ---+------------+----------- > > ro | X | Yes > > ---+------------+----------- > > > > Yes: is supported > > No: is not supported > > X: is not feasible > > I think they are all supported. You can use rw mode with backward > direction but it's just not intuitive. Also ro mode with forward direction > is working but there's a chance to miss the start position of the > previous event. I am a bit confused for the all four modes are supported. >From the code [1], we can see there have only two combinations: - overwrite + read-only mode (PROT_READ); - no-overwrite + read-write mode (PROT_READ | PROT_WRITE); And I think perf tool must use the backward writing for "overwrite", and use the forward writing for "non-overwrite" mode. I think the code [2] can help us to conclude this. In the end, the perf tool doesn't support the combinations: - overwrite + read-write mode - no-overwrite + read-only mode I have sent a v6 patch, please let me know if you have any concerns or if I misunderstand anything. Thanks! Leo [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/evlist.c#n474 [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/mmap.c#n141
