On 11/06, Jens Axboe wrote:
>
> On Tue, Nov 06 2007, Oleg Nesterov wrote:
> >
> > (I guess this patch is not complete, overflow is still possible)
>
> Hmm, where would it overflow?
What if the queue was idle long enough (no !CLASS_IDLE requests) so that
jiffies wraps into the past wrt ->last_end_request?
IOW, perhaps something like the patch below makes some sense. Of course,
this is only a theoretical problem, I'm not sure we really need a fix.
Oleg.
[PATCH] cfq_idle_class_timer: add paranoid checks for jiffies overflow
In theory, if the queue was idle long enough, cfq_idle_class_timer may have
a false (and very long) timeout because jiffies can wrap into the past wrt
->last_end_request.
Signed-off-by: Oleg Nesterov <[email protected]>
--- cfq/block/cfq-iosched.c~ 2007-11-07 15:04:44.000000000 +0300
+++ cfq/block/cfq-iosched.c 2007-11-07 16:04:08.000000000 +0300
@@ -789,6 +789,20 @@ static inline void cfq_slice_expired(str
__cfq_slice_expired(cfqd, cfqq, timed_out);
}
+static int start_idle_class_timer(struct cfq_data *cfqd)
+{
+ unsigned long end = cfqd->last_end_request + CFQ_IDLE_GRACE;
+ unsigned long now = jiffies;
+
+ if (time_before(now, end) &&
+ time_after_eq(now, cfqd->last_end_request)) {
+ mod_timer(&cfqd->idle_class_timer, end);
+ return 1;
+ }
+
+ return 0;
+}
+
/*
* Get next queue for service. Unless we have a queue preemption,
* we'll simply select the first cfqq in the service tree.
@@ -805,19 +819,14 @@ static struct cfq_queue *cfq_get_next_qu
cfqq = rb_entry(n, struct cfq_queue, rb_node);
if (cfq_class_idle(cfqq)) {
- unsigned long end;
-
/*
* if we have idle queues and no rt or be queues had
* pending requests, either allow immediate service if
* the grace period has passed or arm the idle grace
* timer
*/
- end = cfqd->last_end_request + CFQ_IDLE_GRACE;
- if (time_before(jiffies, end)) {
- mod_timer(&cfqd->idle_class_timer, end);
+ if (start_idle_class_timer(cfqd))
cfqq = NULL;
- }
}
return cfqq;
@@ -2033,17 +2042,14 @@ out_cont:
static void cfq_idle_class_timer(unsigned long data)
{
struct cfq_data *cfqd = (struct cfq_data *) data;
- unsigned long flags, end;
+ unsigned long flags;
spin_lock_irqsave(cfqd->queue->queue_lock, flags);
/*
* race with a non-idle queue, reset timer
*/
- end = cfqd->last_end_request + CFQ_IDLE_GRACE;
- if (!time_after_eq(jiffies, end))
- mod_timer(&cfqd->idle_class_timer, end);
- else
+ if (!start_idle_class_timer(cfqd))
cfq_schedule_dispatch(cfqd);
spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
-
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