Re: [PATCH v1] sched/cputime: Preserve iowait accounting across waking interrupts in dyntick-idle

[Christian Loehle <christian.loehle@arm.com>]

On 10/14/26 02:52, Wei Gao wrote:
> In liburing's test suite, the test/iowait.t test case intermittently fails
> with:
>     iowait diff too small: 48
>     test 1 failed
> on tickless systems where IRQ time accounting (CONFIG_IRQ_TIME_ACCOUNTING)
> is not enabled.
> 
> Commit 7198e3927a14 ("sched/cputime: Handle idle irqtime gracefully")
> optimized dyntick-idle cputime accounting by avoiding pausing idle
> accounting on IRQ entry when IRQ time accounting is disabled.
> 
> However, this causes iowait time to be erroneously accounted as
> CPUTIME_IDLE if the task waiting for I/O is woken up inside the interrupt
> handler (such as the 1-second timeout hrtimer in liburing's test/iowait.t):
> 
> 1. In test/iowait.t, a read request is submitted on an empty pipe and
>    io_uring_wait_cqe_timeout() is called with a 1-second timeout. Because
>    current->in_iowait is set, rq->nr_iowait on the CPU is incremented
>    to 1. When the CPU enters tickless idle, the periodic scheduler tick
>    is stopped.
> 2. When the 1-second timeout expires, the hrtimer interrupt fires.
>    Because irqtime is disabled, kcpustat_irq_enter() does not pause idle
>    accounting.
> 3. Inside the interrupt handler, io_cqring_timer_wakeup() calls
>    wake_up_process(). In ttwu_do_activate(), waking up the task immediately
>    decrements rq->nr_iowait from 1 to 0 before the CPU has exited idle.
> 4. When the CPU returns from the interrupt and calls tick_nohz_idle_exit()
>    -> kcpustat_idle_stop() to settle the elapsed tickless idle duration,
>    it checks:
>        iowait = nr_iowait_cpu(smp_processor_id()) > 0;
>    Because rq->nr_iowait was already decremented by ttwu_do_activate(),
>    this evaluates to false. As a result, the entire 1-second idle duration
>    spent waiting for I/O is credited to CPUTIME_IDLE instead of
>    CPUTIME_IOWAIT, causing diff < 50 and failing the test.

So reading the test, the reason this doesn't fail constantly (on a non-isolated
machine) is the massive margins?
It pins the io_uring_wait_cqe_timeout() to CPU0 with either in_iowait=1 or
in_iowait=0.
It then asserts after 1s that CPU0 got: 
a) for in_iowait=0 at most +10 iowait units (<10% CPU time)
b) for in_iowait=1 at least +50 iowait units (>50% CPU time)
So a) would fail on too many unrelated iowaits, b) would fail on unrelated
runnable tasks.

I don't think validating the per-task in_iowait setting via the CPU aggregate
is really feasible FWIW.

> 
> Fix this by checking nr_iowait_cpu() in kcpustat_irq_enter(). If the CPU is
> currently in iowait, pause and settle the elapsed time to CPUTIME_IOWAIT
> before the interrupt handler runs. On kcpustat_irq_exit(), resume
> accounting if it was paused.
> 
> Fixes: 7198e3927a14 ("sched/cputime: Handle idle irqtime gracefully")
> Signed-off-by: Wei Gao <wegao@suse.com>
> ---
>  kernel/sched/cputime.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 06bddaa738e5..e8b56433c61c 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -496,7 +496,8 @@ void kcpustat_irq_enter(u64 now)
>  	struct kernel_cpustat *kc = kcpustat_this_cpu;
>  
>  	if (!vtime_generic_enabled_this_cpu() &&
> -	    (irqtime_enabled() || vtime_accounting_enabled_this_cpu()))
> +	    (irqtime_enabled() || vtime_accounting_enabled_this_cpu() ||
> +	     nr_iowait_cpu(smp_processor_id()) > 0))
>  		kcpustat_idle_stop(kc, now);
>  }
>  
> @@ -510,9 +511,13 @@ void kcpustat_irq_exit(u64 now)
>  	 * need to pause nohz accounting. Resume nohz accounting as long
>  	 * as the irqtime config is enabled to handle case where irqtime
>  	 * accounting got runtime disabled in the middle of an IRQ.
> +	 *
> +	 * Also resume accounting if it was paused on IRQ enter due to
> +	 * iowait.
>  	 */
>  	if (!vtime_generic_enabled_this_cpu() &&
> -	    (IS_ENABLED(CONFIG_IRQ_TIME_ACCOUNTING) || vtime_accounting_enabled_this_cpu()))
> +	    (IS_ENABLED(CONFIG_IRQ_TIME_ACCOUNTING) || vtime_accounting_enabled_this_cpu() ||
> +	     !kc->idle_elapse))
>  		kcpustat_idle_start(kc, now);
>  }
>