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

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Date:   Tue, 20 Apr 2021 09:32:14 +0200
From:   Christian König <christian.koenig@....com>
To:     Michal Hocko <mhocko@...e.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

Am 20.04.21 um 09:04 schrieb Michal Hocko:
> On Mon 19-04-21 18:37:13, Christian König wrote:
>> Am 19.04.21 um 18:11 schrieb Michal Hocko:
> [...]
>>> 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.
>> No, not at all. The pages of a single DMA-buf could even be from different
>> NUMA nodes if the exporting driver decides that this is somehow useful.
> As the use of the counter hasn't been explained yet I can only
> speculate. One thing that I can imagine to be useful is to fill gaps in
> our accounting. It is quite often that the memroy accounted in
> /proc/meminfo (or oom report) doesn't add up to the overall memory
> usage. In some workloads the workload can be huge! In many cases there
> are other means to find out additional memory by a subsystem specific
> interfaces (e.g. networking buffers). I do assume that dma-buf is just
> one of those and the counter can fill the said gap at least partially
> for some workloads. That is definitely useful.

Yes, completely agree. I'm just not 100% sure if the DMA-buf framework 
should account for that or the individual drivers exporting DMA-bufs.

See below for a further explanation.

> What I am trying to bring up with NUMA side is that the same problem can
> happen on per-node basis. Let's say that some user consumes unexpectedly
> large amount of dma-buf on a certain node. This can lead to observable
> performance impact on anybody on allocating from that node and even
> worse cause an OOM for node bound consumers. How do I find out that it
> was dma-buf that has caused the problem?

Yes, that is the direction my thinking goes as well, but also even further.

See DMA-buf is also used to share device local memory between processes 
as well. In other words VRAM on graphics hardware.

On my test system here I have 32GB of system memory and 16GB of VRAM. I 
can use DMA-buf to allocate that 16GB of VRAM quite easily which then 
shows up under /proc/meminfo as used memory.

But that isn't really system memory at all, it's just allocated device 
memory.

> See where I am heading?

Yeah, totally. Thanks for pointing this out.

Suggestions how to handle that?

Regards,
Christian.