Re: [PATCH bpf-next v5 1/3] bpf: Parameterize task iterators.

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sat, Aug 13, 2022 at 3:17 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 8/10/22 5:16 PM, Kui-Feng Lee wrote:
> > Allow creating an iterator that loops through resources of one task/thread.
> >
> > People could only create iterators to loop through all resources of
> > files, vma, and tasks in the system, even though they were interested
> > in only the resources of a specific task or process.  Passing the
> > additional parameters, people can now create an iterator to go
> > through all resources or only the resources of a task.
> >
> > Signed-off-by: Kui-Feng Lee <kuifeng@fb.com>
> > ---
> >   include/linux/bpf.h            |  29 ++++++++
> >   include/uapi/linux/bpf.h       |   8 +++
> >   kernel/bpf/task_iter.c         | 126 ++++++++++++++++++++++++++-------
> >   tools/include/uapi/linux/bpf.h |   8 +++
> >   4 files changed, 147 insertions(+), 24 deletions(-)
> >

Btw, Yonghong, I tried to figure it out myself, but got lost in all
the kernel functions that don't seem to be very well documented. Sorry
for being lazy and throwing this at you :)

Is it easy and efficient to iterate only processes using whatever
kernel helpers we have at our disposal? E.g., if I wanted to write an
iterator that would go only over processes (not individual threads,
just task leaders of each different process) within a cgroup, is that
possible?

I see task iterator as consisting of two different parts (and that
makes it a bit hard to define nice and clean interface, but if we can
crack this, we'd get an elegant and powerful construct):

1. What entity to iterate: threads or processes? (I'm ignoring
task_vma and task_files here, but one could task about files of each
thread or files of each process, but it's less practical, probably)

2. What's the scope of objects to iterate: just a thread by tid, just
a process by pid/pidfd, once cgroup iter lands, we'll be able to talk
about threads or processes within a cgroup or cgroup hierarchy (this
is where descendants_{pre,post}, cgroup_self_only and ancestors
ordering comes in as well).

Currently Kui-Feng is addressing first half of #2 (tid/pid/pidfd
parameters), we can use cgroup iter's parameters to define the scope
of tasks/processes by cgroup "filter" in a follow up (it naturally
extends what we have in this patch set).

So now I'm wondering if there is any benefit to also somehow
specifying threads vs processes as entities to iterate? And if we do
that, does kernel support efficient iteration of processes (as opposed
to threads).


To be clear, there is a lot of value in having just #2, but while we
are all at this topic, I thought I'd clarify for myself #1 as well.

Thanks!