linux-kernel - Re: [PATCH v3] genirq: add support for warning on long-running IRQ handlers

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Message-ID: <87ldodrkcl.ffs@tglx>
Date: Thu, 24 Jul 2025 11:47:38 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jiri Slaby <jirislaby@...nel.org>, Wladislav Wiebe
 <wladislav.wiebe@...ia.com>, corbet@....net, jirislaby@...nel.org
Cc: akpm@...ux-foundation.org, paulmck@...nel.org, rostedt@...dmis.org,
 Neeraj.Upadhyay@....com, david@...hat.com, bp@...en8.de, arnd@...db.de,
 fvdl@...gle.com, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 peterz@...radead.org
Subject: Re: [PATCH v3] genirq: add support for warning on long-running IRQ
 handlers

On Thu, Jul 24 2025 at 07:18, Jiri Slaby wrote:

> On 23. 07. 25, 20:28, Wladislav Wiebe wrote:
>> Introduce a mechanism to detect and warn about prolonged IRQ handlers.
>> With a new command-line parameter (irqhandler.duration_warn_us=),
>> users can configure the duration threshold in microseconds when a warning
>> in such format should be emitted:
>> 
>> "[CPU14] long duration of IRQ[159:bad_irq_handler [long_irq]], took: 1330 us"
>> 
>> The implementation uses local_clock() to measure the execution duration of the
>> generic IRQ per-CPU event handler.
> ...> +static inline void irqhandler_duration_check(u64 ts_start, 
> unsigned int irq,
>> +					     const struct irqaction *action)
>> +{
>> +	/* Approx. conversion to microseconds */
>> +	u64 delta_us = (local_clock() - ts_start) >> 10;
>
> Is this a microoptimization -- have you measured what speedup does it 
> bring? IOW is it worth it instead of cleaner "/ NSEC_PER_USEC"?

A 64-bit division is definitely more expensive than a shift operation
and on 32-bit w/o a 64-bit divide instruction it's more than horribly
slow.

> Or instead, you could store the diff in irqhandler_duration_threshold_ns 
> (mind that "_ns") and avoid the shift and div completely.

That's the right thing to do. The setup code can do a *1000 and be done.

> And what about the wrap? Don't you need abs_diff()?

~500 years after boot :)

Thanks,

        tglx