Re: [Linaro-mm-sig] [PATCH v3] dmabuf: Add the capability to expose DMA-BUF stats in sysfs

[Christian König]

Am 28.01.21 um 12:00 schrieb Sumit Semwal:
> Hi Hridya,
>
> On Wed, 27 Jan 2021 at 17:36, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Tue, Jan 26, 2021 at 12:42:36PM -0800, 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.
> > >
> > > 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 improvements over the previous version:
> > > 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@xxxxxxxxxx>
> > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Thanks for the patch!
>
> Christian: If you're satisfied with the explanation around not
> directly embedding kobjects into the dma_buf and dma_buf_attachment
> structs, then with Greg's r-b from sysfs PoV, I think we can merge it.
> Please let me know if you feel otherwise!

From the technical side it looks clean to me, feel free to add my 
acked-by while pushing.

But I would at least try to convince Daniel on the design. At least some 
of his concerns seems to be valid and keep in mind that we need to 
support this interface forever.

Regards,
Christian.

> > > ---
> > > Changes in v3:
> > > Fix a warning reported by the kernel test robot.
> > >
> > > Changes in v2:
> > > -Move statistics to /sys/kernel/dmabuf/buffers in oder to allow addition
> > > of other DMA-BUF-related sysfs stats in future. Based on feedback from
> > > Daniel Vetter.
> > > -Each attachment has its own directory to represent attaching devices as
> > > symlinks and to introduce map_count as a separate file. Based on
> > > feedback from Daniel Vetter and Christian König. Thank you both!
> > > -Commit messages updated to point to userspace code in AOSP that will
> > > read the DMA-BUF sysfs stats.
> > >
> > >
> > >   .../ABI/testing/sysfs-kernel-dmabuf-buffers   |  52 ++++
> > >   drivers/dma-buf/Kconfig                       |  11 +
> > >   drivers/dma-buf/Makefile                      |   1 +
> > >   drivers/dma-buf/dma-buf-sysfs-stats.c         | 285 ++++++++++++++++++
> > >   drivers/dma-buf/dma-buf-sysfs-stats.h         |  62 ++++
> > >   drivers/dma-buf/dma-buf.c                     |  37 +++
> > >   include/linux/dma-buf.h                       |  20 ++
> > >   7 files changed, 468 insertions(+)
> > >   create mode 100644 Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
> > >   create mode 100644 drivers/dma-buf/dma-buf-sysfs-stats.c
> > >   create mode 100644 drivers/dma-buf/dma-buf-sysfs-stats.h
> > I don't know the dma-buf code at all, but from a sysfs/kobject point of
> > view, this patch looks good to me:
> >
> > Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Best,
> Sumit.
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