@@ -2085,11 +2085,24 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
+ if (p->in_iowait) {
+ delayacct_blkio_end();
+ atomic_dec(&task_rq(p)->nr_iowait);
+ }
+
cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
if (task_cpu(p) != cpu) {
wake_flags |= WF_MIGRATED;
set_task_cpu(p, cpu);
}
+
+#else /* CONFIG_SMP */
+
+ if (p->in_iowait) {
+ delayacct_blkio_end();
+ atomic_dec(&task_rq(p)->nr_iowait);
+ }
+
#endif /* CONFIG_SMP */
ttwu_queue(p, cpu, wake_flags);
@@ -2139,8 +2152,13 @@ static void try_to_wake_up_local(struct task_struct *p, struct pin_cookie cookie
trace_sched_waking(p);
- if (!task_on_rq_queued(p))
+ if (!task_on_rq_queued(p)) {
+ if (p->in_iowait) {
+ delayacct_blkio_end();
+ atomic_dec(&task_rq(p)->nr_iowait);
+ }
ttwu_activate(rq, p, ENQUEUE_WAKEUP);
+ }
ttwu_do_wakeup(rq, p, 0, cookie);
ttwu_stat(p, smp_processor_id(), 0);
@@ -2948,6 +2966,36 @@ unsigned long long nr_context_switches(void)
return sum;
}
+/*
+ * IO-wait accounting, and how its mostly bollocks (on SMP).
+ *
+ * The idea behind IO-wait account is to account the idle time that we could
+ * have spend running if it were not for IO. That is, if we were to improve the
+ * storage performance, we'd have a proportional reduction in IO-wait time.
+ *
+ * This all works nicely on UP, where, when a task blocks on IO, we account
+ * idle time as IO-wait, because if the storage were faster, it could've been
+ * running and we'd not be idle.
+ *
+ * This has been extended to SMP, by doing the same for each CPU. This however
+ * is broken.
+ *
+ * Imagine for instance the case where two tasks block on one CPU, only the one
+ * CPU will have IO-wait accounted, while the other has regular idle. Even
+ * though, if the storage were faster, both could've ran at the same time,
+ * utilising both CPUs.
+ *
+ * This means, that when looking globally, the current IO-wait accounting on
+ * SMP is a lower bound, by reason of under accounting.
+ *
+ * Worse, since the numbers are provided per CPU, they are sometimes
+ * interpreted per CPU, and that is nonsensical. A blocked task isn't strictly
+ * associated with any one particular CPU, it can wake to another CPU than it
+ * blocked on. This means the per CPU IO-wait number is meaningless.
+ *
+ * Task CPU affinities can make all that even more 'interesting'.
+ */
+
unsigned long nr_iowait(void)
{
unsigned long i, sum = 0;
@@ -2958,6 +3006,13 @@ unsigned long nr_iowait(void)
return sum;
}
+/*
+ * Consumers of these two interfaces, like for example the cpufreq menu
+ * governor are using nonsensical data. Boosting frequency for a CPU that has
+ * IO-wait which might not even end up running the task when it does become
+ * runnable.
+ */
+
unsigned long nr_iowait_cpu(int cpu)
{
struct rq *this = cpu_rq(cpu);
@@ -3369,6 +3424,11 @@ static void __sched notrace __schedule(bool preempt)
deactivate_task(rq, prev, DEQUEUE_SLEEP);
prev->on_rq = 0;
+ if (prev->in_iowait) {
+ atomic_inc(&rq->nr_iowait);
+ delayacct_blkio_start();
+ }
+
/*
* If a worker went to sleep, notify and ask workqueue
* whether it wants to wake up a task to maintain