[RFC PATCH 3/5] cfq-iosched: reimplement priorities using different service trees

[RFC PATCH 3/5] cfq-iosched: reimplement priorities using different service trees

[Corrado Zoccolo]

We use different service trees for different priority classes.
This allows a simplification in the service tree insertion code, that no
longer has to consider priority while walking the tree.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
---
 block/cfq-iosched.c |   94 ++++++++++++++++++++++++++++++++++-----------------
 1 files changed, 63 insertions(+), 31 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index f866b17..a5a1786 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -121,6 +121,12 @@ struct cfq_queue {
 	struct cfq_io_context *cic;
 };
 
+enum wl_prio_t {
+	IDLE_WL = -1,
+	BE_WL = 0,
+	RT_WL = 1
+};
+
 /*
  * Per block device queue structure
  */
@@ -130,7 +136,10 @@ struct cfq_data {
 	/*
 	 * rr list of queues with requests and the count of them
 	 */
-	struct cfq_rb_root service_tree;
+	struct cfq_rb_root service_trees[2];
+	struct cfq_rb_root service_tree_idle;
+
+	enum wl_prio_t serving_prio;
 
 	/*
 	 * Each priority tree is sorted by next_request position.  These
@@ -140,7 +149,6 @@ struct cfq_data {
 	struct rb_root prio_trees[CFQ_PRIO_LISTS];
 
 	unsigned int busy_queues;
-	unsigned int busy_rt_queues;
 	unsigned int busy_queues_avg[2];
 
 	int rq_in_driver[2];
@@ -193,6 +201,13 @@ struct cfq_data {
 	unsigned long last_end_sync_rq;
 };
 
+static struct cfq_rb_root *service_tree_for(enum wl_prio_t prio,
+					struct cfq_data *cfqd)
+{
+	return prio == IDLE_WL ? &cfqd->service_tree_idle :
+		&cfqd->service_trees[prio];
+}
+
 enum cfqq_state_flags {
 	CFQ_CFQQ_FLAG_on_rr = 0,	/* on round-robin busy list */
 	CFQ_CFQQ_FLAG_wait_request,	/* waiting for a request */
@@ -237,6 +252,12 @@ CFQ_CFQQ_FNS(coop);
 #define cfq_log(cfqd, fmt, args...)	\
 	blk_add_trace_msg((cfqd)->queue, "cfq " fmt, ##args)
 
+static inline int cfq_busy_queues_wl(enum wl_prio_t wl, struct cfq_data *cfqd)
+{
+	return wl == IDLE_WL ? cfqd->service_tree_idle.count :
+		cfqd->service_trees[wl].count;
+}
+
 static void cfq_dispatch_insert(struct request_queue *, struct request *);
 static struct cfq_queue *cfq_get_queue(struct cfq_data *, bool,
 				       struct io_context *, gfp_t);
@@ -315,8 +336,7 @@ cfq_get_avg_queues(struct cfq_data *cfqd, bool rt) {
 	unsigned min_q, max_q;
 	unsigned mult  = cfq_hist_divisor - 1;
 	unsigned round = cfq_hist_divisor / 2;
-	unsigned busy  = rt ? cfqd->busy_rt_queues :
-		(cfqd->busy_queues - cfqd->busy_rt_queues);
+	unsigned busy  = cfq_busy_queues_wl(rt, cfqd);
 	min_q = min(cfqd->busy_queues_avg[rt], busy);
 	max_q = max(cfqd->busy_queues_avg[rt], busy);
 	cfqd->busy_queues_avg[rt] = (mult * max_q + min_q + round) /
@@ -520,7 +540,7 @@ static unsigned long cfq_slice_offset(struct cfq_data *cfqd,
 }
 
 /*
- * The cfqd->service_tree holds all pending cfq_queue's that have
+ * The cfqd->service_trees holds all pending cfq_queue's that have
  * requests waiting to be processed. It is sorted in the order that
  * we will service the queues.
  */
@@ -530,11 +550,12 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 	struct rb_node **p, *parent;
 	struct cfq_queue *__cfqq;
 	unsigned long rb_key;
-	struct cfq_rb_root *service_tree = &cfqd->service_tree;
+	struct cfq_rb_root *service_tree;
 	int left;
 
 	if (cfq_class_idle(cfqq)) {
 		rb_key = CFQ_IDLE_DELAY;
+		service_tree = &cfqd->service_tree_idle;
 		parent = rb_last(&service_tree->rb);
 		if (parent && parent != &cfqq->rb_node) {
 			__cfqq = rb_entry(parent, struct cfq_queue, rb_node);
@@ -542,6 +563,8 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		} else
 			rb_key += jiffies;
 	} else if (!add_front) {
+		enum wl_prio_t prio = cfq_class_rt(cfqq) ? RT_WL : BE_WL;
+		service_tree = service_tree_for(prio, cfqd);
 		/*
 		 * Get our rb key offset. Subtract any residual slice
 		 * value carried from last service. A negative resid
@@ -552,6 +575,8 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		rb_key -= cfqq->slice_resid;
 		cfqq->slice_resid = 0;
 	} else {
+		enum wl_prio_t prio = cfq_class_rt(cfqq) ? RT_WL : BE_WL;
+		service_tree = service_tree_for(prio, cfqd);
 		rb_key = -HZ;
 		__cfqq = cfq_rb_first(service_tree);
 		rb_key += __cfqq ? __cfqq->rb_key : jiffies;
@@ -561,7 +586,8 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		/*
 		 * same position, nothing more to do
 		 */
-		if (rb_key == cfqq->rb_key)
+		if (rb_key == cfqq->rb_key &&
+		    cfqq->service_tree == service_tree)
 			return;
 
 		cfq_rb_erase(&cfqq->rb_node, cfqq->service_tree);
@@ -579,25 +605,14 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		__cfqq = rb_entry(parent, struct cfq_queue, rb_node);
 
 		/*
-		 * sort RT queues first, we always want to give
-		 * preference to them. IDLE queues goes to the back.
-		 * after that, sort on the next service time.
+		 * sort by key, that represents service time.
 		 */
-		if (cfq_class_rt(cfqq) > cfq_class_rt(__cfqq))
-			n = &(*p)->rb_left;
-		else if (cfq_class_rt(cfqq) < cfq_class_rt(__cfqq))
-			n = &(*p)->rb_right;
-		else if (cfq_class_idle(cfqq) < cfq_class_idle(__cfqq))
-			n = &(*p)->rb_left;
-		else if (cfq_class_idle(cfqq) > cfq_class_idle(__cfqq))
-			n = &(*p)->rb_right;
-		else if (time_before(rb_key, __cfqq->rb_key))
+		if (time_before(rb_key, __cfqq->rb_key))
 			n = &(*p)->rb_left;
-		else
+		else {
 			n = &(*p)->rb_right;
-
-		if (n == &(*p)->rb_right)
 			left = 0;
+		}
 
 		p = n;
 	}
@@ -696,8 +711,7 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	BUG_ON(cfq_cfqq_on_rr(cfqq));
 	cfq_mark_cfqq_on_rr(cfqq);
 	cfqd->busy_queues++;
-	if (cfq_class_rt(cfqq))
-		cfqd->busy_rt_queues++;
+
 	cfq_resort_rr_list(cfqd, cfqq);
 }
 
@@ -722,8 +736,6 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 
 	BUG_ON(!cfqd->busy_queues);
 	cfqd->busy_queues--;
-	if (cfq_class_rt(cfqq))
-		cfqd->busy_rt_queues--;
 }
 
 /*
@@ -977,10 +989,12 @@ static inline void cfq_slice_expired(struct cfq_data *cfqd, bool timed_out)
  */
 static struct cfq_queue *cfq_get_next_queue(struct cfq_data *cfqd)
 {
-	if (RB_EMPTY_ROOT(&cfqd->service_tree.rb))
-		return NULL;
+	struct cfq_rb_root *service_tree =
+		service_tree_for(cfqd->serving_prio, cfqd);
 
-	return cfq_rb_first(&cfqd->service_tree);
+	if (RB_EMPTY_ROOT(&service_tree->rb))
+		return NULL;
+	return cfq_rb_first(service_tree);
 }
 
 /*
@@ -1098,6 +1112,10 @@ static struct cfq_queue *cfq_close_cooperator(struct cfq_data *cfqd,
 	if (cfq_cfqq_coop(cfqq))
 		return NULL;
 
+	/* we don't want to mix processes with different characteristics */
+	if (cfqq->service_tree != cur_cfqq->service_tree)
+		return NULL;
+
 	if (!probe)
 		cfq_mark_cfqq_coop(cfqq);
 	return cfqq;
@@ -1264,6 +1282,14 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
 expire:
 	cfq_slice_expired(cfqd, 0);
 new_queue:
+	if (!new_cfqq) {
+		if (cfq_busy_queues_wl(RT_WL, cfqd))
+			cfqd->serving_prio = RT_WL;
+		else if (cfq_busy_queues_wl(BE_WL, cfqd))
+			cfqd->serving_prio = BE_WL;
+		else
+			cfqd->serving_prio = IDLE_WL;
+	}
 	cfqq = cfq_set_active_queue(cfqd, new_cfqq);
 keep_queue:
 	return cfqq;
@@ -1290,8 +1316,12 @@ static int cfq_forced_dispatch(struct cfq_data *cfqd)
 {
 	struct cfq_queue *cfqq;
 	int dispatched = 0;
+	int i;
+	for (i = 0; i < 2; ++i)
+		while ((cfqq = cfq_rb_first(&cfqd->service_trees[i])) != NULL)
+			dispatched += __cfq_forced_dispatch_cfqq(cfqq);
 
-	while ((cfqq = cfq_rb_first(&cfqd->service_tree)) != NULL)
+	while ((cfqq = cfq_rb_first(&cfqd->service_tree_idle)) != NULL)
 		dispatched += __cfq_forced_dispatch_cfqq(cfqq);
 
 	cfq_slice_expired(cfqd, 0);
@@ -2538,7 +2568,9 @@ static void *cfq_init_queue(struct request_queue *q)
 	if (!cfqd)
 		return NULL;
 
-	cfqd->service_tree = CFQ_RB_ROOT;
+	for (i = 0; i < 2; ++i)
+		cfqd->service_trees[i] = CFQ_RB_ROOT;
+	cfqd->service_tree_idle = CFQ_RB_ROOT;
 
 	/*
 	 * Not strictly needed (since RB_ROOT just clears the node and we
-- 
1.6.2.5

Re: [RFC PATCH 3/5] cfq-iosched: reimplement priorities using different service trees

[Jens Axboe]

On Mon, Oct 19 2009, Corrado Zoccolo wrote:
> We use different service trees for different priority classes.
> This allows a simplification in the service tree insertion code, that no
> longer has to consider priority while walking the tree.

> +enum wl_prio_t {
> +	IDLE_WL = -1,
> +	BE_WL = 0,
> +	RT_WL = 1
> +};

WL?

> +static struct cfq_rb_root *service_tree_for(enum wl_prio_t prio,
> +					struct cfq_data *cfqd)
> +{
> +	return prio == IDLE_WL ? &cfqd->service_tree_idle :
> +		&cfqd->service_trees[prio];
> +}

Don't do it...

static struct cfq_rb_root *service_tree_for(enum wl_prio_t prio,
					struct cfq_data *cfqd)
{
        if (prio == IDLE_WL)
                return &cfqd->service_tree_idle;

        return &cfqd->service_trees[prio];
}

much cleaner. There are more of these in this patch.

Otherwise it looks sane, and I agree that making the insert cleaner here
is a good bonus.

-- 
Jens Axboe

Re: [RFC PATCH 3/5] cfq-iosched: reimplement priorities using different service trees

[Corrado Zoccolo]

On Tue, Oct 20, 2009 at 2:58 AM, Jens Axboe <jens.axboe@oracle.com> wrote:
> On Mon, Oct 19 2009, Corrado Zoccolo wrote:
>> We use different service trees for different priority classes.
>> This allows a simplification in the service tree insertion code, that no
>> longer has to consider priority while walking the tree.
>
>> +enum wl_prio_t {
>> +     IDLE_WL = -1,
>> +     BE_WL = 0,
>> +     RT_WL = 1
>> +};
>
> WL?
It stands for "workload". When defining all-caps constants, one has to
avoid possible clashes adding pre/suf-fixes.
Any suggestion for better naming?
>
>> +static struct cfq_rb_root *service_tree_for(enum wl_prio_t prio,
>> +                                     struct cfq_data *cfqd)
>> +{
>> +     return prio == IDLE_WL ? &cfqd->service_tree_idle :
>> +             &cfqd->service_trees[prio];
>> +}
>
> Don't do it...
>
> static struct cfq_rb_root *service_tree_for(enum wl_prio_t prio,
>                                        struct cfq_data *cfqd)
> {
>        if (prio == IDLE_WL)
>                return &cfqd->service_tree_idle;
>
>        return &cfqd->service_trees[prio];
> }
>
> much cleaner. There are more of these in this patch.
I don't see much difference: my brain just translates the former in
the latter, and the former takes up less screen space.
But you are the maintainer, so I'll write it as you want.

> Otherwise it looks sane, and I agree that making the insert cleaner here
> is a good bonus.

Thanks
Corrado
>
> --
> Jens Axboe
>
>

Re: [RFC PATCH 3/5] cfq-iosched: reimplement priorities using different service trees

[Jens Axboe]

On Tue, Oct 20 2009, Corrado Zoccolo wrote:
> On Tue, Oct 20, 2009 at 2:58 AM, Jens Axboe <jens.axboe@oracle.com> wrote:
> > On Mon, Oct 19 2009, Corrado Zoccolo wrote:
> >> We use different service trees for different priority classes.
> >> This allows a simplification in the service tree insertion code, that no
> >> longer has to consider priority while walking the tree.
> >
> >> +enum wl_prio_t {
> >> +     IDLE_WL = -1,
> >> +     BE_WL = 0,
> >> +     RT_WL = 1
> >> +};
> >
> > WL?
> It stands for "workload". When defining all-caps constants, one has to
> avoid possible clashes adding pre/suf-fixes.
> Any suggestion for better naming?

Nobody will guess that. Make variable/enum names as short as possible,
but not so short that their meaning are incomprehensible. Suggestion
would be to use IDLE_WORKLOAD etc.

-- 
Jens Axboe