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

[Christian König]

Am 10.12.20 um 11:56 schrieb Greg KH:
> On Thu, Dec 10, 2020 at 11:27:27AM +0100, Daniel Vetter wrote:
> > On Thu, Dec 10, 2020 at 11:10:45AM +0100, Greg KH wrote:
> > > On Thu, Dec 10, 2020 at 10:58:50AM +0100, Christian König wrote:
> > > > In general a good idea, but I have a few concern/comments here.
> > > >
> > > > Am 10.12.20 um 05:43 schrieb Hridya Valsaraju:
> > > > > 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/<inode_number>/exporter_name
> > > > > /sys/kernel/dmabuf/<inode_number>/size
> > > > > /sys/kernel/dmabuf/<inode_number>/dev_map_info
> > > > >
> > > > > 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.
> > > > Mhm, this makes it part of the UAPI. What is the justification for this?
> > > >
> > > > In other words do we really need those debug information in a production
> > > > environment?
> > > Production environments seem to want to know who is using up memory :)
> > This only shows shared memory, so it does smell a lot like $specific_issue
> > and we're designing a narrow solution for that and then have to carry it
> > forever.
> I think the "issue" is that this was a feature from ion that people
> "missed" in the dmabuf move.  Taking away the ability to see what kind
> of allocations were being made didn't make a lot of debugging tools
> happy :(

Yeah, that is certainly a very valid concern.

> But Hridya knows more, she's been dealing with the transition for a long
> time now.
>
> > E.g. why is the list of attachments not a sysfs link? That's how we
> > usually expose struct device * pointers in sysfs to userspace, not as a
> > list of things.
> These aren't struct devices, so I don't understand the objection here.
> Where else could these go in sysfs?

Sure they are! Just take a look at an attachment:

struct dma_buf_attachment {
         struct dma_buf *dmabuf;
         struct device *dev;

This is the struct device which is importing the buffer and the patch in 
discussion is just printing the name of this device into sysfs.

> > Furthermore we don't have the exporter device covered anywhere, how is
> > that tracked? Yes Android just uses ion for all shared buffers, but that's
> > not how all of linux userspace works.
> Do we have the exporter device link in the dmabuf interface?  If so,
> great, let's use that, but for some reason I didn't think it was there.

Correct, since we don't really need a device as an exporter (it can just 
be a system heap as well) we only have a const char* as name for the 
exporter.

> > Then I guess there's the mmaps, you can fish them out of procfs. A tool
> > which collects all that information might be useful, just as demonstration
> > of how this is all supposed to be used.
> There's a script somewhere that does this today, again, Hridya knows
> more.
>
> > There's also some things to make sure we're at least having thought about
> > how other things fit in here. E.d. dma_resv attached to the dma-buf
> > matters in general a lot. It doesn't matter on Android because
> > everything's pinned all the time anyway.
> >
> > Also I thought sysfs was one value one file, dumping an entire list into
> > dev_info_map with properties we'll need to extend (once you care about
> > dma_resv you also want to know which attachments are dynamic) does not
> > smell like sysfs design at all.
> sysfs is one value per file, what is being exported that is larger than
> that here?  Did I miss something on review?

See this chunk here:

+
+    list_for_each_entry(attachment, &dmabuf->attachments, node) {
+        if (attachment->map_counter) {
+            ret += sysfs_emit_at(buf, ret, "%s ",
+                         dev_name(attachment->dev));
+        }
+    }

And yes now that Daniel mentioned that it looks like a sysfs rules 
violation to me as well.

Regards,
Christian.


> thanks,
>
> greg k-h