Re: [patch 2/3]cfq-iosched: schedule dispatch for noidle queue

[Shaohua Li <shaohua.li@intel.com>]

On Tue, 2010-11-09 at 10:15 +0800, Vivek Goyal wrote:
> On Tue, Nov 09, 2010 at 09:31:07AM +0800, Shaohua Li wrote:
> > On Mon, 2010-11-08 at 22:28 +0800, Vivek Goyal wrote:
> > > On Mon, Nov 08, 2010 at 10:07:18AM +0800, Shaohua Li wrote:
> > > > A queue is idle at cfq_dispatch_requests(), but it gets noidle later. 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/block/cfq-iosched.c
> > > > ===================================================================
> > > > --- linux.orig/block/cfq-iosched.c	2010-11-08 08:41:20.000000000 +0800
> > > > +++ linux/block/cfq-iosched.c	2010-11-08 08:43:51.000000000 +0800
> > > > @@ -3265,6 +3265,10 @@ cfq_should_preempt(struct cfq_data *cfqd
> > > >  	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
> > > >  		return true;
> > > >  
> > > > +	/* An idle queue should not be idle now for some reason */
> > > > +	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;
> > > >  
> > > > @@ -3508,8 +3512,25 @@ static void cfq_completed_request(struct
> > > >  		}
> > > >  	}
> > > >  
> > > > -	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. 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
> > > > +	 * */
> > > 
> > > Why do we have to wait for all requests to finish in device? Will driver
> > > most likely not ask for next request when 1-2 requests have completed
> > > and at that time we should expire the queue if queue is no more marked
> > > as "noidle"?
> > The issue is a queue is idle just because it's the last queue of the
> > service tree. Then a new queue is added and the idled queue should not
> > idle now. we should preempt the idled queue soon. does this make sense
> > to you?
> 
> If that's the case then you should just modify should_preempt() so that
> addition of a new queue could preempt an empty queue which has now become
> noidle.
> 
> You have also modified cfq_completed_request() function, which will wake
> up the worker thread and then try to dispatch a request. IMHO, in practice
> driver asks for new request almost immediately and you don't gain much
> by this additional wakeup.
> 
> So my point being, that we increased the code complexity for no visible
> performance improvement also increased thread wakeups resulting in more
> cpu consumption.
Ah, you are right, we only need modify should_preempt. Updated the patch as below.

> If there was a visible performance gain in your testing then it would have
> made sense but you said that you did not notice any improvements. Then
> why to increase the complexity.
I only test some workloads and can't do all tests, but this is an
obvious bug I thought.

Thanks,
Shaohua

Subject: cfq-iosched: preempt an idle queue if it should not be idle any more

A queue is idle at cfq_dispatch_requests(), but it gets noidle later. 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 |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux/block/cfq-iosched.c
===================================================================
--- linux.orig/block/cfq-iosched.c	2010-11-09 10:20:38.000000000 +0800
+++ linux/block/cfq-iosched.c	2010-11-09 10:20:54.000000000 +0800
@@ -3265,6 +3265,10 @@ cfq_should_preempt(struct cfq_data *cfqd
 	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
 		return true;
 
+	/* An idle queue should not be idle now for some reason */
+	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;