[PATCH v4] dmabuf: Add the capability to expose DMA-BUF stats in sysfs

[Greg KH]

On Tue, May 25, 2021 at 11:37:13AM -0700, Hridya Valsaraju wrote:
> This patch allows statistics to be enabled for each DMA-BUF in
> sysfs by enabling the config CONFIG_DMABUF_SYSFS_STATS.
> 
> The following stats will be exposed by the interface:
> 
> /sys/kernel/dmabuf/buffers/<inode_number>/exporter_name
> /sys/kernel/dmabuf/buffers/<inode_number>/size
> /sys/kernel/dmabuf/buffers/<inode_number>/attachments/<attach_uid>/device
> /sys/kernel/dmabuf/buffers/<inode_number>/attachments/<attach_uid>/map_counter
> 
> The inode_number is unique for each DMA-BUF and was added earlier [1]
> in order to allow userspace to track DMA-BUF usage across different
> processes.
> 
> Currently, this information is exposed in
> /sys/kernel/debug/dma_buf/bufinfo.
> However, since debugfs is considered unsafe to be mounted in production,
> it is being duplicated in sysfs.
> 
> Procfs also uses the proc/<pid>/fdinfo/<fd> file to expose some
> information about DMA-BUF fds. However, the existing procfs interfaces
> can only provide information about the buffers for which processes
> hold fds or have the buffers mmapped into their address space.
> The sysfs interface also exposes attachment statistics for each buffer.
> 
> This information will be used to derive DMA-BUF
> per-exporter stats and per-device usage stats for Android Bug reports.
> The corresponding userspace changes can be found at [2].
> Telemetry tools will also capture this information(along with other
> memory metrics) periodically as well as on important events like a
> foreground app kill (which might have been triggered by Low Memory
> Killer). It will also contribute to provide a snapshot of the system
> memory usage on other events such as OOM kills and Application Not
> Responding events.
> 
> A shell script that can be run on a classic Linux environment to read
> out the DMA-BUF statistics can be found at [3](suggested by John
> Stultz).
> 
> The patch contains the following major improvements over v1:
> 1) Each attachment is represented by its own directory to allow creating
> a symlink to the importing device and to also provide room for future
> expansion.
> 2) The number of distinct mappings of each attachment is exposed in a
> separate file.
> 3) The per-buffer statistics are now in /sys/kernel/dmabuf/buffers
> inorder to make the interface expandable in future.
> 
> All of the improvements above are based on suggestions/feedback from
> Daniel Vetter and Christian König.
> 
> [1]: https://lore.kernel.org/patchwork/patch/1088791/
> [2]: https://android-review.googlesource.com/q/topic:%22dmabuf-sysfs%22+(status:open%20OR%20status:merged)
> [3]: https://android-review.googlesource.com/c/platform/system/memory/libmeminfo/+/1549734
> 
> Signed-off-by: Hridya Valsaraju <hridya at google.com>
> Reported-by: kernel test robot <lkp at intel.com>
> ---

Reviewed-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>