On Tue, Aug 22, 2023 at 7:38 PM Leo Yan <leo.yan@xxxxxxxxxx> wrote: > > Hi Namhyung, > > On Tue, Aug 22, 2023 at 05:43:25PM -0700, Namhyung Kim wrote: > > [...] > > > > +2.3.2 Writing samples into buffer > > > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > > + > > > +Ring buffers are mapped as read-write mode or read-only mode, which is > > > +used for a normal ring buffer and an overwritable ring buffer > > > +respectively. > > > + > > > +The ring buffer in the read-write mode is mapped with the property > > > +``PROT_READ | PROT_WRITE``. With the write permission, the perf tool > > > +updates the ``data_tail`` to indicate the data start position. Combining > > > +with the head pointer ``data_head``, which works as the end position of > > > +the current data, the perf tool can easily know where read out the data > > > +from. > > > + > > > +Alternatively, in the read-only mode, only the kernel keeps to update > > > +the ``data_head`` while the user space cannot access the ``data_tail`` due > > > +to the mapping property ``PROT_READ``. > > > + > > > +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. Thanks, Namhyung > > > And I think the description below in this section shows kernel > > internals too much. Name of kernel functions and data structure > > is not an API and can be changed any time. You can describe > > the logic without the names. > > Understand, I agree this will introduce maintenance efforts in later. > > I will refine the description and send a new patch. > > Thanks for reviewing! > > Leo
