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

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Date:   Thu, 10 Dec 2020 13:07:32 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Daniel Vetter <daniel@...ll.ch>
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 12:26:01PM +0100, Daniel Vetter wrote:
> 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.

Back "over where"?

dma-bufs are not only used for the graphics stack on android from what I
can tell, so this shouldn't be a gpu-specific issue.

confused,

greg k-h