[PATCH 2/4] bpf: Add dmabuf iterator

[Song Liu]

On Wed, Apr 16, 2025 at 4:40 PM T.J. Mercier <tjmercier at google.com> wrote:
>
> On Wed, Apr 16, 2025 at 4:08 PM Song Liu <song at kernel.org> wrote:
> >
> > On Wed, Apr 16, 2025 at 3:51 PM T.J. Mercier <tjmercier at google.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

Thanks for sharing this information.

> 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.

I haven't tried drgn on Android. On server side, iterating should 200
cgroups should be fairly fast (< 5 seconds, where DWARF parsing is
the most expensive part).

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

Here is another rookie question, it appears to me there is a file descriptor
associated with each DMA buffer, can we achieve the same goal with
a task-file iterator?

Thanks,
Song