[PATCH v2 1/4] sched: move IO scheduling accounting from io_schedule_timeout() to __schedule()

[Tejun Heo <tj@kernel.org>]

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
schedule().

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
__schedule() so that later patches can separate out preparation and
finish steps from io_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>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavan Kondeti <pkondeti@codeaurora.org>
---
 kernel/sched/core.c |   23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3335,12 +3335,18 @@ static void __sched notrace __schedule(b
 	struct task_struct *prev, *next;
 	unsigned long *switch_count;
 	struct pin_cookie cookie;
-	struct rq *rq;
-	int cpu;
+	struct rq *rq, *prev_rq;
+	int cpu, in_iowait;
 
 	cpu = smp_processor_id();
-	rq = cpu_rq(cpu);
+	rq = prev_rq = cpu_rq(cpu);
 	prev = rq->curr;
+	in_iowait = prev->in_iowait;
+
+	if (in_iowait) {
+		delayacct_blkio_start();
+		atomic_inc(&rq->nr_iowait);
+	}
 
 	schedule_debug(prev);
 
@@ -3406,6 +3412,11 @@ static void __sched notrace __schedule(b
 	}
 
 	balance_callback(rq);
+
+	if (in_iowait) {
+		atomic_dec(&prev_rq->nr_iowait);
+		delayacct_blkio_end();
+	}
 }
 
 void __noreturn do_task_dead(void)
@@ -5063,19 +5074,13 @@ 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;
 }