@@ -2085,11 +2085,24 @@ try_to_wake_up(struct task_struct *p, un
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
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(&rq->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(v
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(b
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
@@ -5063,19 +5123,13 @@ EXPORT_SYMBOL_GPL(yield_to);
long __sched io_schedule_timeout(long timeout)
{
int old_iowait = current->in_iowait;
- struct rq *rq;
long ret;
current->in_iowait = 1;
blk_schedule_flush_plug(current);
- delayacct_blkio_start();
- rq = raw_rq();
- atomic_inc(&rq->nr_iowait);
ret = schedule_timeout(timeout);
current->in_iowait = old_iowait;
- atomic_dec(&rq->nr_iowait);
- delayacct_blkio_end();
return ret;
}
Hello, Yeah, that works. Here's v3 based on your patch. The other patches still apply correctly. Thanks. ------ 8< ------ For an interface to support blocking for IOs, it must call io_schedule() instead of schedule(). This makes it tedious to add IO blocking to existing interfaces as the switching between schedule() and io_schedule() is often buried deep. As we already have a way to mark the task as IO scheduling, this can be made easier by separating out io_schedule() into multiple steps so that IO schedule preparation can be performed before invoking a blocking interface and the actual accounting happens inside the scheduler. io_schedule_timeout() does the following three things prior to calling schedule_timeout(). 1. Mark the task as scheduling for IO. 2. Flush out plugged IOs. 3. Account the IO scheduling. #1 and #2 can be performed in the prepartaion step while #3 must be done close to the actual scheduling. This patch moves #3 into the scheduler so that later patches can separate out preparation and finish steps from io_schedule(). v3: Replaced with PeterZ's implementation which performs nr_iowait accounting in the sleep and wake up path to avoid unnecessarily burdening non sleeping paths in __schedule(). v2: Remember the rq in @prev_rq and use it for decrementing nr_iowait to avoid misattributing the count after the task gets migrated to another CPU. Noticed by Pavan. Signed-off-by: Tejun Heo <tj@kernel.org> Patch-originally-by: Peter Zijlstra <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Pavan Kondeti <pkondeti@codeaurora.org> --- kernel/sched/core.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 61 insertions(+), 7 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html