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

[Wei Gao <wegao@suse.com>]

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.

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);
 }
 
-- 
2.55.0