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Message-ID: <tip-e33a9bba85a869b85fd0a7f16e21a5ec8977e325@git.kernel.org>
Date: Sat, 14 Jan 2017 04:49:25 -0800
From: tip-bot for Tejun Heo <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: torvalds@...ux-foundation.org, tglx@...utronix.de, tj@...nel.org,
mingo@...nel.org, efault@....de, linux-kernel@...r.kernel.org,
peterz@...radead.org, hpa@...or.com
Subject: [tip:sched/core] sched/core: move IO scheduling accounting from
io_schedule_timeout() into scheduler
Commit-ID: e33a9bba85a869b85fd0a7f16e21a5ec8977e325
Gitweb: http://git.kernel.org/tip/e33a9bba85a869b85fd0a7f16e21a5ec8977e325
Author: Tejun Heo <tj@...nel.org>
AuthorDate: Wed, 7 Dec 2016 15:48:41 -0500
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Sat, 14 Jan 2017 11:30:03 +0100
sched/core: move IO scheduling accounting from io_schedule_timeout() into scheduler
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.
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().
Patch-originally-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: adilger.kernel@...ger.ca
Cc: akpm@...ux-foundation.org
Cc: axboe@...nel.dk
Cc: jack@...e.com
Cc: kernel-team@...com
Cc: mingbo@...com
Cc: tytso@....edu
Link: http://lkml.kernel.org/r/20161207204841.GA22296@htj.duckdns.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
kernel/sched/core.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 61 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 96a4267..9fd3716 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2089,11 +2089,24 @@ 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);
@@ -2143,8 +2156,13 @@ static void try_to_wake_up_local(struct task_struct *p, struct rq_flags *rf)
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, rf);
ttwu_stat(p, smp_processor_id(), 0);
@@ -2956,6 +2974,36 @@ 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;
@@ -2966,6 +3014,13 @@ 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);
@@ -3377,6 +3432,11 @@ 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
@@ -5075,19 +5135,13 @@ 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;
}
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