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

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Message-ID: <cd9f9c65-4416-4a9e-91e7-572bc25266f9@kernel.org>
Date: Thu, 24 Jul 2025 07:30:57 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Wladislav Wiebe <wladislav.wiebe@...ia.com>, tglx@...utronix.de,
 corbet@....net
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 24. 07. 25, 7:18, 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"?
> 
> Or instead, you could store the diff in irqhandler_duration_threshold_ns 
> (mind that "_ns") and avoid the shift and div completely.
> 
> And what about the wrap? Don't you need abs_diff()?

Not that ^^^, it won't work, but something else. But if I am counting 
correctly, the wrap is in 584 years if counted from 0. Well, for 
native/tsc, "Intel guarantees that the time-stamp counter will not 
wraparound within 10 years after being reset". I have no idea what 
virtualizations return to local_clock(). This is not my call to decide, 
though.

> thanks,
-- 
js
suse labs