linux-kernel - Re: [PATCH v2 7/7] mm/tracing: introduce max_vma_count

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Message-ID: <6x64jf7szyy52gug6qoixhhjq6vsrcqpozqj4ambsehh2jprj2@qeye6qevem4g>
Date: Thu, 18 Sep 2025 12:38:56 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Kalesh Singh <kaleshsingh@...gle.com>, akpm@...ux-foundation.org, 
	minchan@...nel.org, lorenzo.stoakes@...cle.com, david@...hat.com, 
	Liam.Howlett@...cle.com, rppt@...nel.org, kernel-team@...roid.com, android-mm@...gle.com, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Kees Cook <kees@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, 
	Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Jann Horn <jannh@...gle.com>, Shuah Khan <shuah@...nel.org>, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 7/7] mm/tracing: introduce max_vma_count_exceeded
 trace event

On Tue, Sep 16, 2025 at 11:52:20AM -0400, Steven Rostedt wrote:
> On Mon, 15 Sep 2025 18:19:53 -0700
> Kalesh Singh <kaleshsingh@...gle.com> wrote:
> 
> 
> > Hi Steve,
> > 
> > Thanks for the comments and suggestion you are right we can use bpf to
> > get the comm. There is nothing special about this trace event.  I will
> > drop comm in the next revision.
> > 
> > The reason I did the task_struct parameter (current): I believe there
> > is a limitation that we must  specify at least 1 parameter to the
> > TRACE_EVENT()  PROTO and ARGS macros.
> 
> OK, then this is another issue. We don't want tracepoint "markers".
> Each tracepoint can take up to 5K in memory due to the code it
> generates and the meta data to control it.
> 
> For something like that, we highly recommend dynamic probes (fprobes,
> kprobes, etc).
> 
> The only purpose of a static tracepoint is to get data within a
> function that is too difficult to get via a probe. It should never be
> used as a trigger where its purpose is "we hit this path".
>

Isn't the usual problem with that approach, that of static functions/static
inline functions? I was tracing through a problem a few months ago, and man
I really did think "wouldn't it be nice to have a tracepoint instead of fishing
around for kprobe spots".

Not that I particularly think a tracepoint is super worth it in this case, but,
y'know.

-- 
Pedro