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