linux-kernel - Re: [PATCH v2] genirq/manage: Reduce priority of forced secondary interrupt handler

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Message-ID: <aQDIxvU18vzB-1G-@wunner.de>
Date: Tue, 28 Oct 2025 14:44:38 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Thomas Gleixner <tglx@...utronix.de>, Crystal Wood <crwood@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org, Attila Fazekas <afazekas@...hat.com>,
	linux-pci@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Bjorn Helgaas <helgaas@...nel.org>,
	Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
	Oliver OHalloran <oohall@...il.com>
Subject: Re: [PATCH v2] genirq/manage: Reduce priority of forced secondary
 interrupt handler

On Tue, Oct 28, 2025 at 01:06:52PM +0100, Sebastian Andrzej Siewior wrote:
> On 2025-10-27 13:59:31 [+0100], Lukas Wunner wrote:
> > The issue does not show on non-PREEMPT_RT because the primary handler
> > runs in hardirq context and thus can preempt the threaded secondary
> > handler, clear the Root Error Status register and prevent the secondary
> > handler from getting stuck.
> 
> Not sure if I mentioned it before but this is due to forced threaded
> IRQs which can also be enabled on non-PREEMPT_RT systems via `threadirqs`.

According to the commit which introduced the "threadirqs" command line
option, 8d32a307e4fa ("genirq: Provide forced interrupt threading"),
it is "mostly a debug option".  I guess the option allows testing
the waters on arches which do not yet "select ARCH_SUPPORTS_RT"
to see if force-threaded interrupts break anything.  I recall the
option being available in mainline for much longer than PREEMPT_RT
and it was definitely useful as a justification to upstream changes
which were otherwise only needed by the out-of-tree PREEMPT_RT patches.

Intuitively I would assume that debug options are not worth calling out
in commit messages or code comments as users and developers will
primarily be interested in the real deal (i.e. PREEMPT_RT) and not
an option which gets us only halfway there.  However if you
(or anyone else) feels strongly about it, I'll be happy to respin.

Thanks for taking a look!

Lukas