Re: [PATCH 1/4] mm/damon/dbgfs: Implement recording feature

On Mon, 11 Oct 2021 09:30:57 +0000 SeongJae Park [off-list ref] wrote:

Hello Andrew,


Thank you for great questions!

On Sun, 10 Oct 2021 15:01:40 -0700 Andrew Morton [off-list ref] wrote:

quoted

On Fri,  8 Oct 2021 09:45:06 +0000 SeongJae Park [off-list ref] wrote:

quoted

The user space can get the monitoring results via the 'damon_aggregated'
tracepoint event.  For simplicity and brevity, the tracepoint events
have some duplicated information such as 'target_id' and 'nr_regions',
though.  As a result, its size is greater than really needed.  Also,
dealing with the tracepoint could be complex for some simple use cases.
To provide a way for getting more efficient and simple monitoring
results to user space, this commit implements 'recording' feature in
'damon-dbgfs'.

The feature is exported to the user space via a new debugfs file named
'record', which is located in '<debugfs>/damon/' directory.  The file
allows users to record monitored access patterns in a regular binary
file in a simple format.

Binary files are troublesome.

Is the format of this file documented anywhere?

No.  I intended the Python script in the following patch[1] and the user space
tool[2] to be used as such documents.  I will write up one before the next
spin.

[1] https://lore.kernel.org/linux-mm/20211008094509.16179-3-sj@kernel.org/ (local)
[2] https://github.com/awslabs/damo/blob/v0.0.5/_damon_result.py#L38

quoted

I assume that the file's contents will have different representations
depending on host endianness and word size and I further assume that
the provided python script won't handle this very well?

You're right.  I will make the script properly handle the cases in the next
spin.

Well, rather than messing with the different file formats, you could
make the binary file machine-independent.  Decide on the endianness and
word size, implement them and document them.  Things like cpu_to_le32
are zero-cost on little-endian machines.