Re: [PATCH] kunit: irq: Continue increasing hrtimer interval for longer

["Ard Biesheuvel" <ardb@kernel.org>]

On Mon, 3 Aug 2026, at 21:18, Eric Biggers wrote:
> Currently, kunit_irq_test_timer_func() stops increasing the hrtimer
> interval as soon as some forward progress is made in each of softirq and
> task context.  Update it to use a more aggressive strategy: increase the
> interval as long as the hrtimer is running significantly faster than
> either context.
>
> This resolves an occasional hang in the CRC and crypto library tests
> under qemu-system-s390x.  It was exposed by the change in the default
> preemption model on s390 from NONE to LAZY.  That seems to have exposed
> the issue by allowing some forward progress to be made while the actual
> system timer tick is still starved, preventing jiffies from increasing
> or the task context from making much progress towards max_iterations.
>
> Fixes: 201ceb94aa1d ("kunit: irq: Ensure timer doesn't fire too frequently")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ---
>
> This patch is targeting libcrypto-next
>
>  include/kunit/run-in-irq-context.h | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/include/kunit/run-in-irq-context.h 
> b/include/kunit/run-in-irq-context.h
> index bfe60d6cf28d..3802b6fb218e 100644
> --- a/include/kunit/run-in-irq-context.h
> +++ b/include/kunit/run-in-irq-context.h
> @@ -38,11 +38,13 @@ static enum hrtimer_restart 
> kunit_irq_test_timer_func(struct hrtimer *timer)
>  	softirq_calls = atomic_read(&state->softirq_func_calls);
> 
>  	/*
> -	 * If the timer is firing too often for the softirq or task to ever have
> -	 * a chance to run, increase the timer interval.  This is needed on very
> -	 * slow systems.
> +	 * If the hrtimer is running much faster than the bh_work or the task,
> +	 * then it is firing too fast and might be starving those contexts as
> +	 * well as the actual system timer tick.  Increase the interval.
>  	 */
> -	if (hardirq_calls >= 20 && (softirq_calls == 0 || task_calls == 0))
> +	if (hardirq_calls >= 20 &&
> +	    (hardirq_calls / 2 > softirq_calls ||
> +	     hardirq_calls / 2 > task_calls))
>  		state->interval = ktime_add_ns(state->interval, 250);
> 
>  	if (!state->func(state->test_specific_state))
>

Acked-by: Ard Biesheuvel <ardb@kernel.org>