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

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Message-ID: <20251028135756.31PG0uHY@linutronix.de>
Date: Tue, 28 Oct 2025 14:57:56 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Lukas Wunner <lukas@...ner.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 2025-10-28 14:44:38 [+0100], Lukas Wunner wrote:
> 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.

There are people using it without PREEMPT_RT due to $reasons. It is not
documented in Documentation/admin-guide/kernel-parameters.txt as an
option meant only for debugging.

> 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.

I argued and sent patches to fix code which was wrong on PREEMPT_RT due
to threadirqs and was also wrong without PREEMPT_RT enabled but solely
with forced-threaded irqs.

But please wait once tglx/irq or peterz/sched says something here before
repining.

> Thanks for taking a look!
> 
> Lukas

Sebastian