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Date: Thu, 22 Feb 2024 15:16:12 +0100
From: Michal Hocko <mhocko@suse.com>
To: Carlos Galo <carlosgalo@google.com>
Cc: rostedt@goodmis.org, akpm@linux-foundation.org, surenb@google.com,
	android-mm@google.com, kernel-team@android.com,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v2] mm: Update mark_victim tracepoints fields
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References: <20240111210539.636607-1-carlosgalo@google.com>
 <ZdWshXSoLthv6J6b@tiehlicka>
 <CABtOLR+gpU2BYxcGCCqccZGcRDF337z3JJ=7nvC47ANHmS1tNA@mail.gmail.com>
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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.

> > > 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?
 
> > 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.
-- 
Michal Hocko
SUSE Labs