linux-kernel - Re: [PATCH v4] dma-buf: Add DmaBufTotal counter in meminfo

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Message-ID: <YH2ru642wYfqK5ne@dhcp22.suse.cz>
Date:   Mon, 19 Apr 2021 18:11:39 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Christian König <christian.koenig@....com>
Cc:     Peter.Enderborg@...y.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, sumit.semwal@...aro.org,
        adobriyan@...il.com, akpm@...ux-foundation.org,
        songmuchun@...edance.com, guro@...com, shakeelb@...gle.com,
        neilb@...e.de, samitolvanen@...gle.com, rppt@...nel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, willy@...radead.org
Subject: Re: [PATCH v4] dma-buf: Add DmaBufTotal counter in meminfo

On Mon 19-04-21 17:44:13, Christian König wrote:
> Am 19.04.21 um 17:19 schrieb Peter.Enderborg@...y.com:
> > On 4/19/21 5:00 PM, Michal Hocko wrote:
> > > On Mon 19-04-21 12:41:58, Peter.Enderborg@...y.com wrote:
> > > > On 4/19/21 2:16 PM, Michal Hocko wrote:
> > > > > On Sat 17-04-21 12:40:32, Peter Enderborg wrote:
> > > > > > This adds a total used dma-buf memory. Details
> > > > > > can be found in debugfs, however it is not for everyone
> > > > > > and not always available. dma-buf are indirect allocated by
> > > > > > userspace. So with this value we can monitor and detect
> > > > > > userspace applications that have problems.
> > > > > The changelog would benefit from more background on why this is needed,
> > > > > and who is the primary consumer of that value.
> > > > > 
> > > > > I cannot really comment on the dma-buf internals but I have two remarks.
> > > > > Documentation/filesystems/proc.rst needs an update with the counter
> > > > > explanation and secondly is this information useful for OOM situations
> > > > > analysis? If yes then show_mem should dump the value as well.
> > > > > 
> > > > >  From the implementation point of view, is there any reason why this
> > > > > hasn't used the existing global_node_page_state infrastructure?
> > > > I fix doc in next version.  Im not sure what you expect the commit message to include.
> > > As I've said. Usual justification covers answers to following questions
> > > 	- Why do we need it?
> > > 	- Why the existing data is insuficient?
> > > 	- Who is supposed to use the data and for what?
> > > 
> > > I can see an answer for the first two questions (because this can be a
> > > lot of memory and the existing infrastructure is not production suitable
> > > - debugfs). But the changelog doesn't really explain who is going to use
> > > the new data. Is this a monitoring to raise an early alarm when the
> > > value grows? Is this for debugging misbehaving drivers? How is it
> > > valuable for those?
> > > 
> > > > The function of the meminfo is: (From Documentation/filesystems/proc.rst)
> > > > 
> > > > "Provides information about distribution and utilization of memory."
> > > True. Yet we do not export any random counters, do we?
> > > 
> > > > Im not the designed of dma-buf, I think  global_node_page_state as a kernel
> > > > internal.
> > > It provides a node specific and optimized counters. Is this a good fit
> > > with your new counter? Or the NUMA locality is of no importance?
> > Sounds good to me, if Christian Koenig think it is good, I will use that.
> > It is only virtio in drivers that use the global_node_page_state if
> > that matters.
> 
> DMA-buf are not NUMA aware at all. On which node the pages are allocated
> (and if we use pages at all and not internal device memory) is up to the
> exporter and importer.

The question is not whether it is NUMA aware but whether it is useful to
know per-numa data for the purpose the counter is supposed to serve.
-- 
Michal Hocko
SUSE Labs