linux-kernel - Re: [RFT] sched_ext: Skip stack trace capture in NMI context

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Message-ID: <aUoCBVhHig737uKt@slm.duckdns.org>
Date: Mon, 22 Dec 2025 16:44:21 -1000
From: Tejun Heo <tj@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, David Vernet <void@...ifault.com>,
	Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.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>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, sched-ext@...ts.linux.dev
Subject: Re: [RFT] sched_ext: Skip stack trace capture in NMI context

Hello,

On Mon, Dec 22, 2025 at 07:50:37PM -0500, Joel Fernandes wrote:
> stack_trace_save() is not guaranteed to be NMI-safe on all
> architectures.
> 
> The hardlockup detector calls into sched_ext via the following call
> chain when an NMI occurs:
> 
>   watchdog_overflow_callback()
>     watchdog_hardlockup_check()
>       scx_hardlockup()
>         stack_trace_save()
> 
> Skip stack trace capture when in_nmi() returns true to prevent
> potential deadlocks.
> 
> Fixes: 582f700e1bdc ("sched_ext: Hook up hardlockup detector")
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>

This does work on x86 (right?) and is useful in understanding what the
underlying problem is. It'd be great if there's a config flag we can test
but if not can we specifically exclude archs which are known to not work?

Thanks.

-- 
tejun