[patch]cfq-iosched: schedule dispatch for noidle queue

[patch]cfq-iosched: schedule dispatch for noidle queue

[Shaohua Li]

A queue is idle at cfq_dispatch_requests(), but it gets noidle later for
different reasons. Unless other task explictly does unplug or all requests
are drained, we will not deliever requests to the disk even cfq_arm_slice_timer
doesn't make the queue idle. For example, cfq_should_idle() returns true
because of service_tree->count == 1, and then other queues are added.
Note, I didn't see obvious performance impacts so far with the patch, but
just thought this could be a problem.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

---
 block/cfq-iosched.c |   23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

Index: linux-2.6/block/cfq-iosched.c
===================================================================
--- linux-2.6.orig/block/cfq-iosched.c	2010-09-21 00:20:46.000000000 +0800
+++ linux-2.6/block/cfq-iosched.c	2010-09-21 00:22:58.000000000 +0800
@@ -3256,6 +3256,10 @@
 	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
 		return true;
 
+	/* An idle queue should not be idle now for different reasons */
+	if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq))
+		return true;
+
 	if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
 		return false;
 
@@ -3509,8 +3513,25 @@
 		}
 	}
 
-	if (!cfqd->rq_in_driver)
+	if (!cfqd->rq_in_driver) {
+		cfq_schedule_dispatch(cfqd);
+		return;
+	}
+	/*
+	 * A queue is idle at cfq_dispatch_requests(), but it gets noidle later
+	 * for different reasons. We schedule a dispatch if the queue has no
+	 * requests, otherwise the disk is actually in idle till all requests
+	 * are finished even cfq_arm_slice_timer doesn't make the queue idle
+	 * */
+	cfqq = cfqd->active_queue;
+	if (!cfqq)
+		return;
+
+	if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq) &&
+	    (!cfqd->cfq_group_idle || cfqq->cfqg->nr_cfqq > 1)) {
+		cfq_del_timer(cfqd, cfqq);
 		cfq_schedule_dispatch(cfqd);
+	}
 }
 
 /*

Re: [patch]cfq-iosched: schedule dispatch for noidle queue

[Vivek Goyal]

On Mon, Sep 20, 2010 at 04:53:37PM +0800, Shaohua Li wrote:
> A queue is idle at cfq_dispatch_requests(), but it gets noidle later for
> different reasons. Unless other task explictly does unplug or all requests
> are drained, we will not deliever requests to the disk even cfq_arm_slice_timer
> doesn't make the queue idle. For example, cfq_should_idle() returns true
> because of service_tree->count == 1, and then other queues are added.
> Note, I didn't see obvious performance impacts so far with the patch, but
> just thought this could be a problem.
> 

Shaohua Li,

What are those reasons when an idling queue becomes a non idle one
(without getting a new request in the queue)?

Vivek

> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> 
> ---
>  block/cfq-iosched.c |   23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/block/cfq-iosched.c
> ===================================================================
> --- linux-2.6.orig/block/cfq-iosched.c	2010-09-21 00:20:46.000000000 +0800
> +++ linux-2.6/block/cfq-iosched.c	2010-09-21 00:22:58.000000000 +0800
> @@ -3256,6 +3256,10 @@
>  	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
>  		return true;
>  
> +	/* An idle queue should not be idle now for different reasons */
> +	if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq))
> +		return true;
> +
>  	if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
>  		return false;
>  
> @@ -3509,8 +3513,25 @@
>  		}
>  	}
>  
> -	if (!cfqd->rq_in_driver)
> +	if (!cfqd->rq_in_driver) {
> +		cfq_schedule_dispatch(cfqd);
> +		return;
> +	}
> +	/*
> +	 * A queue is idle at cfq_dispatch_requests(), but it gets noidle later
> +	 * for different reasons. We schedule a dispatch if the queue has no
> +	 * requests, otherwise the disk is actually in idle till all requests
> +	 * are finished even cfq_arm_slice_timer doesn't make the queue idle
> +	 * */
> +	cfqq = cfqd->active_queue;
> +	if (!cfqq)
> +		return;
> +
> +	if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq) &&
> +	    (!cfqd->cfq_group_idle || cfqq->cfqg->nr_cfqq > 1)) {
> +		cfq_del_timer(cfqd, cfqq);
>  		cfq_schedule_dispatch(cfqd);
> +	}
>  }
>  
>  /*
> 

Re: [patch]cfq-iosched: schedule dispatch for noidle queue

[Shaohua Li]

On Mon, 2010-09-20 at 22:02 +0800, Vivek Goyal wrote:
> On Mon, Sep 20, 2010 at 04:53:37PM +0800, Shaohua Li wrote:
> > A queue is idle at cfq_dispatch_requests(), but it gets noidle later for
> > different reasons. Unless other task explictly does unplug or all requests
> > are drained, we will not deliever requests to the disk even cfq_arm_slice_timer
> > doesn't make the queue idle. For example, cfq_should_idle() returns true
> > because of service_tree->count == 1, and then other queues are added.
> > Note, I didn't see obvious performance impacts so far with the patch, but
> > just thought this could be a problem.
> > 
> 
> Shaohua Li,
> 
> What are those reasons when an idling queue becomes a non idle one
> (without getting a new request in the queue)?
OOps, I used to add a new reason for another patch but forgot to update
the comment. So this is the only reason.

Thanks,
Shaohua