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

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Message-ID: <877bxr65aq.ffs@tglx>
Date: Sun, 21 Sep 2025 20:15:09 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Lukas Wunner <lukas@...ner.de>
Cc: Sebastian Andrzej Siewior <bigeasy@...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] genirq/manage: Reduce priority of forced secondary IRQ
 handler

On Sun, Sep 21 2025 at 15:12, Lukas Wunner wrote:
> On Sat, Sep 20, 2025 at 11:20:26PM +0200, Thomas Gleixner wrote:
> Your research that at first glance, at least 21 of 40 instances of
> request_threaded_irq() could just use IRQF_NO_THREAD, seems to
> support the notion that the majority of interrupt handlers only
> do minimal work in hard interrupt context.
>
> But if that is the case, and if you believe that deferring that
> small amount of work to a thread is nonsensical, then why is the
> primary handler forced into a thread by default in the first place,
> requiring drivers to explicitly opt out by setting IRQF_NO_THREAD?

Because there are primary handlers which are absolutely not RT safe.

> Shouldn't it rather be the other way round, i.e. by default the
> primary handler is *not* forced into a thread, but only if the
> driver explicitly opts in?  (In cases where the primary handler
> does a sufficient amount of work that is justified to be deferred
> to a thread.)

We had to do it this way before RT got upstream as there was no way to
play a whack a mole game with drivers constantly being added and
changed.

Thanks,

        tglx