2005-11-09 17:18:10

by Tejun Heo

Subject: [PATCH] blk: cfq forced dispatching fix

cfq forced dispatching might not return all requests on the queue.
This bug can hang elevator switchinig and corrupt request ordering
during flush sequence.

Signed-off-by: Tejun Heo <[email protected]>

---

Jens, I implemented a separate code path for forced dispatching as it
seemed too complex/error-prone to handle forced case in normal
dispatch path. How do you like this approach?

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -999,7 +999,7 @@ cfq_prio_to_maxrq(struct cfq_data *cfqd,
/*
* get next queue for service
*/
-static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd, int force)
+static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
{
unsigned long now = jiffies;
struct cfq_queue *cfqq;
@@ -1023,7 +1023,7 @@ static struct cfq_queue *cfq_select_queu
*/
if (!RB_EMPTY(&cfqq->sort_list))
goto keep_queue;
- else if (!force && cfq_cfqq_class_sync(cfqq) &&
+ else if (cfq_cfqq_class_sync(cfqq) &&
time_before(now, cfqq->slice_end)) {
if (cfq_arm_slice_timer(cfqd, cfqq))
return NULL;
@@ -1092,6 +1092,42 @@ __cfq_dispatch_requests(struct cfq_data
}

static int
+cfq_forced_dispatch_cfqqs(struct list_head *list)
+{
+ int dispatched = 0;
+ struct cfq_queue *cfqq, *next;
+ struct cfq_rq *crq;
+
+ list_for_each_entry_safe(cfqq, next, list, cfq_list) {
+ while ((crq = cfqq->next_crq)) {
+ cfq_dispatch_insert(cfqq->cfqd->queue, crq);
+ dispatched++;
+ }
+ BUG_ON(!list_empty(&cfqq->fifo));
+ }
+ return dispatched;
+}
+
+static int
+cfq_forced_dispatch(struct cfq_data *cfqd)
+{
+ int i, dispatched = 0;
+
+ for (i = 0; i < CFQ_PRIO_LISTS; i++)
+ dispatched += cfq_forced_dispatch_cfqqs(&cfqd->rr_list[i]);
+
+ dispatched += cfq_forced_dispatch_cfqqs(&cfqd->busy_rr);
+ dispatched += cfq_forced_dispatch_cfqqs(&cfqd->cur_rr);
+ dispatched += cfq_forced_dispatch_cfqqs(&cfqd->idle_rr);
+
+ cfq_slice_expired(cfqd, 0);
+
+ BUG_ON(cfqd->busy_queues);
+
+ return dispatched;
+}
+
+static int
cfq_dispatch_requests(request_queue_t *q, int force)
{
struct cfq_data *cfqd = q->elevator->elevator_data;
@@ -1100,7 +1136,10 @@ cfq_dispatch_requests(request_queue_t *q
if (!cfqd->busy_queues)
return 0;

- cfqq = cfq_select_queue(cfqd, force);
+ if (unlikely(force))
+ return cfq_forced_dispatch(cfqd);
+
+ cfqq = cfq_select_queue(cfqd);
if (cfqq) {
int max_dispatch;

@@ -1115,12 +1154,9 @@ cfq_dispatch_requests(request_queue_t *q
cfq_clear_cfqq_wait_request(cfqq);
del_timer(&cfqd->idle_slice_timer);

- if (!force) {
- max_dispatch = cfqd->cfq_quantum;
- if (cfq_class_idle(cfqq))
- max_dispatch = 1;
- } else
- max_dispatch = INT_MAX;
+ max_dispatch = cfqd->cfq_quantum;
+ if (cfq_class_idle(cfqq))
+ max_dispatch = 1;

return __cfq_dispatch_requests(cfqd, cfqq, max_dispatch);
}

2005-11-10 07:46:33

by Jens Axboe

Subject: Re: [PATCH] blk: cfq forced dispatching fix

On Thu, Nov 10 2005, Tejun Heo wrote:
> cfq forced dispatching might not return all requests on the queue.
> This bug can hang elevator switchinig and corrupt request ordering
> during flush sequence.
>
> Signed-off-by: Tejun Heo <[email protected]>
>
> ---
>
> Jens, I implemented a separate code path for forced dispatching as it
> seemed too complex/error-prone to handle forced case in normal
> dispatch path. How do you like this approach?

I like this approach better then mixing the 'force' into various logic
around the function. So applied.

--
Jens Axboe