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

[Wladislav Wiebe <wladislav.wiebe@nokia.com>]

On 24/07/2025 11:47, Thomas Gleixner wrote:
> 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.

Excellent optimization proposal! It has been included in v4: https://lore.kernel.org/lkml/20250724155059.2992-1-wladislav.wiebe@nokia.com/ Thanks, - W.W.