Re: [PATCH v2] mm: Update mark_victim tracepoints fields

[Carlos Galo <carlosgalo@google.com>]

On Thu, Feb 22, 2024 at 6:16 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 21-02-24 13:30:51, Carlos Galo wrote:
> > On Tue, Feb 20, 2024 at 11:55 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > Hi,
> > > sorry I have missed this before.
> > >
> > > On Thu 11-01-24 21:05:30, Carlos Galo wrote:
> > > > The current implementation of the mark_victim tracepoint provides only
> > > > the process ID (pid) of the victim process. This limitation poses
> > > > challenges for userspace tools that need additional information
> > > > about the OOM victim. The association between pid and the additional
> > > > data may be lost after the kill, making it difficult for userspace to
> > > > correlate the OOM event with the specific process.
> > >
> > > You are correct that post OOM all per-process information is lost. On
> > > the other hand we do dump all this information to the kernel log. Could
> > > you explain why that is not suitable for your purpose?
> >
> > Userspace tools often need real-time visibility into OOM situations
> > for userspace intervention. Our use case involves utilizing BPF
> > programs, along with BPF ring buffers, to provide OOM notification to
> > userspace. Parsing kernel logs would be significant overhead as
> > opposed to the event based BPF approach.
>
> Please put that into the changelog.

Will do.

> > > > In order to mitigate this limitation, add the following fields:
> > > >
> > > > - UID
> > > >    In Android each installed application has a unique UID. Including
> > > >    the `uid` assists in correlating OOM events with specific apps.
> > > >
> > > > - Process Name (comm)
> > > >    Enables identification of the affected process.
> > > >
> > > > - OOM Score
> > > >    Allows userspace to get additional insights of the relative kill
> > > >    priority of the OOM victim.
> > >
> > > What is the oom score useful for?
> > >
> > The OOM score provides us a measure of the victim's importance. On the
> > android side, it allows us to identify if top or foreground apps are
> > killed, which have user perceptible impact.
>
> But the value on its own (wihtout knowing scores of other tasks) doesn't
> really tell you anything, does it?

Android uses the OOM adj_score ranges  to categorize app state
(foreground, background, ...). I'll resend a v3 with the commit text
updated to include details about this.

> > > Is there any reason to provide a different information from the one
> > > reported to the kernel log?
> > > __oom_kill_process:
> > > pr_err("%s: Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB, UID:%u pgtables:%lukB oom_score_adj:%hd\n",
> > >                 message, task_pid_nr(victim), victim->comm, K(mm->total_vm),
> > >                 K(get_mm_counter(mm, MM_ANONPAGES)),
> > >                 K(get_mm_counter(mm, MM_FILEPAGES)),
> > >                 K(get_mm_counter(mm, MM_SHMEMPAGES)),
> > >                 from_kuid(&init_user_ns, task_uid(victim)),
> > >                 mm_pgtables_bytes(mm) >> 10, victim->signal->oom_score_adj);
> > >
> >
> > We added these fields we need (UID, process name, and OOM score), but
> > we're open to adding the others if you prefer that for consistency
> > with the kernel log.
>
> yes, I think the consistency would be better here. For one it reports
> numbers which can tell quite a lot about the killed victim. It is a
> superset of what you already asking for. With a notable exception of the
> oom_score which is really dubious without a wider context.

Sounds good, I'll resend a v3 that includes these fields as well.

Thanks,
Carlos

> --
> Michal Hocko
> SUSE Labs