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

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Message-ID: <20250918104800.18682dcb@gandalf.local.home>
Date: Thu, 18 Sep 2025 10:48:00 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Pedro Falcato <pfalcato@...e.de>
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 Thu, 18 Sep 2025 12:38:56 +0100
Pedro Falcato <pfalcato@...e.de> wrote:

> 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.

Yes, it would be useful. The issue is that tracepoints are not free. They
do increase the I$ hit and take up memory.

If you're going to inject a tracepoint somewhere, at least extract some
useful information from that spot. If you can't think of anything to track,
then it's not worth a tracepoint.

If one really wants a way to track something, they could add a static
branch that would call a function when enabled that could be traced. They
would also need a way to enable that static branch.

-- Steve