linux-kernel - Re: [PATCH 2/4] bpf: Add dmabuf iterator

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Message-ID: <CABdmKX1tDv3fSFURDN7=txFSbQ1xTjp8ZhLP8tFAvLcO9_-4_A@mail.gmail.com>
Date: Wed, 16 Apr 2025 16:40:22 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Song Liu <song@...nel.org>
Cc: sumit.semwal@...aro.org, christian.koenig@....com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	skhan@...uxfoundation.org, linux-kernel@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org, 
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, android-mm@...gle.com, 
	simona@...ll.ch, corbet@....net, eddyz87@...il.com, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	jolsa@...nel.org, mykolal@...com
Subject: Re: [PATCH 2/4] bpf: Add dmabuf iterator

On Wed, Apr 16, 2025 at 4:08 PM Song Liu <song@...nel.org> wrote:
>
> On Wed, Apr 16, 2025 at 3:51 PM T.J. Mercier <tjmercier@...gle.com> wrote:
> [...]
> > >
> > > IIUC, the iterator simply traverses elements in a linked list. I feel it is
> > > an overkill to implement a new BPF iterator for it.
> >
> > Like other BPF iterators such as kmem_cache_iter or task_iter.
> > Cgroup_iter iterates trees instead of lists. This is iterating over
> > kernel objects just like the docs say, "A BPF iterator is a type of
> > BPF program that allows users to iterate over specific types of kernel
> > objects". More complicated iteration should not be a requirement here.
> >
> > > Maybe we simply
> > > use debugging tools like crash or drgn for this? The access with
> > > these tools will not be protected by the mutex. But from my personal
> > > experience, this is not a big issue for user space debugging tools.
> >
> > drgn is *way* too slow, and even if it weren't the dependencies for
> > running it aren't available. crash needs debug symbols which also
> > aren't available on user builds. This is not just for manual
> > debugging, it's for reporting memory use in production. Or anything
> > else someone might care to extract like attachment info or refcounts.
>
> Could you please share more information about the use cases and
> the time constraint here, and why drgn is too slow. Is most of the delay
> comes from parsing DWARF? This is mostly for my curiosity, because
> I have been thinking about using drgn to do some monitoring in
> production.
>
> Thanks,
> Song

These RunCommands have 10 second timeouts for example. It's rare that
I see them get exceeded but it happens occasionally.:
https://cs.android.com/android/platform/superproject/main/+/main:frameworks/native/cmds/dumpstate/dumpstate.cpp;drc=98bdc04b7658fde0a99403fc052d1d18e7d48ea6;l=2008

The last time I used drgn (admittedly back in 2023) it took over a
minute to iterate through less than 200 cgroups. I'm not sure what the
root cause of the slowness was, but I'd expect the DWARF processing to
be done up-front once and the slowness I experienced was not just at
startup. Eventually I switched over to tracefs for that issue, which
we still use for some telemetry.

Other uses are by statsd for telemetry, memory reporting on app kills
or death, and for "dumpsys meminfo".

Thanks,
T.J.