Date:   Thu, 10 Dec 2020 12:26:01 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Christian König <christian.koenig@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Android Kernel Team <kernel-team@...roid.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH] dmabuf: Add the capability to expose DMA-BUF stats in sysfs

On Thu, Dec 10, 2020 at 11:55 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> 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 :(

If this is just for dma-heaps then why don't we add the stuff back
over there? It reinforces more that the android gpu stack and the
non-android gpu stack on linux are fairly different in fundamental
ways, but that's not really new.
-Daniel

> 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?
>
> > 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.
>
> > 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?
>
> thanks,
>
> greg k-h
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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