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
> 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
