On 08/20, Peter Zijlstra wrote:
>
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -453,7 +453,8 @@ struct rq {
> u64 clock;
> u64 clock_task;
>
> - atomic_t nr_iowait;
> + int nr_iowait_local;
> + atomic_t nr_iowait_remote;
I am wondering how the extra lock(rq)/unlock(rq) in schedule() is bad
compared to atomic_dec.
IOW, what if we simply make rq->nr_iowait "int" and change schedule()
to update it? Something like below. Just curious.
As for nr_iowait_local + nr_iowait_remote, this doesn't look safe...
in theory nr_iowait_cpu() or even nr_iowait() can return a negative
number.
Oleg.
--- x/kernel/sched/core.c
+++ x/kernel/sched/core.c
@@ -2435,6 +2435,9 @@ need_resched:
rq->curr = next;
++*switch_count;
+ if (unlikely(prev->in_iowait))
+ rq->nr_iowait++;
+
context_switch(rq, prev, next); /* unlocks the rq */
/*
* The context switch have flipped the stack from under us
@@ -2442,6 +2445,12 @@ need_resched:
* this task called schedule() in the past. prev == current
* is still correct, but it can be moved to another cpu/rq.
*/
+ if (unlikely(prev->in_iowait)) {
+ raw_spin_lock_irq(&rq->lock);
+ rq->nr_iowait--;
+ raw_spin_unlock_irq(&rq->lock);
+ }
+
cpu = smp_processor_id();
rq = cpu_rq(cpu);
} else
@@ -3939,31 +3948,24 @@ EXPORT_SYMBOL_GPL(yield_to);
*/
void __sched io_schedule(void)
{
- struct rq *rq = raw_rq();
-
delayacct_blkio_start();
- atomic_inc(&rq->nr_iowait);
blk_flush_plug(current);
current->in_iowait = 1;
schedule();
current->in_iowait = 0;
- atomic_dec(&rq->nr_iowait);
delayacct_blkio_end();
}
EXPORT_SYMBOL(io_schedule);
long __sched io_schedule_timeout(long timeout)
{
- struct rq *rq = raw_rq();
long ret;
delayacct_blkio_start();
- atomic_inc(&rq->nr_iowait);
blk_flush_plug(current);
current->in_iowait = 1;
ret = schedule_timeout(timeout);
current->in_iowait = 0;
- atomic_dec(&rq->nr_iowait);
delayacct_blkio_end();
return ret;
}
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