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

[Wei Gao]

On Wed, Oct 14, 2026 at 03:14:41PM +0100, Christian Loehle wrote:
> 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?
Margins should help, but this case has more chance to failing after
recent kernel change. It does help track kernel behaviro.
> 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@xxxxxxxx>
> > ---
> >  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);
> >  }
> >  
>