Date:	Fri, 17 Dec 2010 17:52:46 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Jarek Poplawski <jarkao2@...il.com>
Subject: [RFC PATCH] net_sched: sch_sfq: better struct layouts

Le jeudi 16 décembre 2010 à 14:08 +0100, Eric Dumazet a écrit :
> Le mercredi 15 décembre 2010 à 18:03 +0100, Patrick McHardy a écrit :
> > On 15.12.2010 17:55, Eric Dumazet wrote:
> 
> > > I was thinking in allowing more packets per SFQ (but keep the 126 active
> > > flows limit), what do you think ?
> > 
> > I keep forgetting why this limit exists, let me try to figure
> > it out once more :)
> 
> I took a look, and found we already are at max, unless we change
> sfq_index type (from u8 to u16)
> 
> SFQ_SLOTS is 128  (not really exist, but this is the number of slots)
> SFQ_DEPTH is 128 (max depth for one slot)
> Constraints are :
>  SFQ_SLOTS < 256
>  SFQ_DEPTH < 256
>  SFQ_SLOTS + SFQ_DEPTH < 256, because of the dep[] array.
> 
> We could avoid the "struct sk_buff_head" structure with its un-needed
> spinlock, and eventually group data for a given slot in a structure to
> reduce number of cache lines we touch...
> 
> struct sfq_slot {
> 	struct sk_buff *first;
> 	struct sk_buff *last;
> 	u8 qlen;
> 	sfq_index next; /* dequeue chain */
> 	u16 hash;
> 	short allot;
> 	/* 16bit hole */
> };
> 
> This would save 768 bytes on x86_64 (and much more if LOCKDEP is used)
> 
> 

Here is a preliminary patch, shrinking sizeof(struct sfq_sched_data)
from 0x14f8 (or more if spinlocks are bigger) to 0x1180 bytes, and
reduce text size as well.

   text	   data	    bss	    dec	    hex	filename
   4821	    152	      0	   4973	   136d	old/net/sched/sch_sfq.o
   4651	    136	      0	   4787	   12b3	new/net/sched/sch_sfq.o


All data for a slot/flow is now grouped in a compact and cache friendly
structure :

struct sfq_slot {
        struct sk_buff  *skblist_next;
        struct sk_buff  *skblist_prev;
        sfq_index       qlen; /* number of skbs in skblist */
        sfq_index       next; /* next slot in sfq chain */
        unsigned short  hash; /* hash value (index in ht[]) */
        short           allot; /* credit for this slot */
        struct sfq_head anchor; /* anchor in dep[] chains */
};



 net/sched/sch_sfq.c |  223 +++++++++++++++++++++++-------------------
 1 file changed, 125 insertions(+), 98 deletions(-)

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 3cf478d..28968eb 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -69,25 +69,40 @@
 	This implementation limits maximal queue length to 128;
 	maximal mtu to 2^15-1; number of hash buckets to 1024.
 	The only goal of this restrictions was that all data
-	fit into one 4K page :-). Struct sfq_sched_data is
-	organized in anti-cache manner: all the data for a bucket
-	are scattered over different locations. This is not good,
-	but it allowed me to put it into 4K.
+	fit into one 4K page on 32bit arches.
 
 	It is easy to increase these values, but not in flight.  */
 
-#define SFQ_DEPTH		128
+#define SFQ_DEPTH		128 /* max number of packets per slot (per flow) */
+#define SFQ_SLOTS		128 /* max number of flows */
+#define EMPTY_SLOT		255
 #define SFQ_HASH_DIVISOR	1024
 
-/* This type should contain at least SFQ_DEPTH*2 values */
+/* This type should contain at least SFQ_DEPTH + SFQ_SLOTS values */
 typedef unsigned char sfq_index;
 
+/*
+ * We dont use pointers to save space.
+ * Small indexes [0 ... SFQ_SLOTS - 1] are 'pointers' to slots[] array
+ * while following values [SFQ_SLOTS ... SFQ_SLOTS + SFQ_DEPTH - 1]
+ * are 'pointers' to dep[] array
+ */
 struct sfq_head
 {
 	sfq_index	next;
 	sfq_index	prev;
 };
 
+struct sfq_slot {
+	struct sk_buff	*skblist_next;
+	struct sk_buff	*skblist_prev;
+	sfq_index	qlen; /* number of skbs in skblist */
+	sfq_index	next; /* next slot in sfq chain */
+	unsigned short	hash; /* hash value (index in ht[]) */
+	short		allot; /* credit for this slot */
+	struct sfq_head anchor; /* anchor in dep[] chains */
+};
+
 struct sfq_sched_data
 {
 /* Parameters */
@@ -99,17 +114,24 @@ struct sfq_sched_data
 	struct tcf_proto *filter_list;
 	struct timer_list perturb_timer;
 	u32		perturbation;
-	sfq_index	tail;		/* Index of current slot in round */
-	sfq_index	max_depth;	/* Maximal depth */
+	sfq_index	max_depth;	/* depth of longest slot */
 
+	struct sfq_slot *tail;		/* current slot in round */
 	sfq_index	ht[SFQ_HASH_DIVISOR];	/* Hash table */
-	sfq_index	next[SFQ_DEPTH];	/* Active slots link */
-	short		allot[SFQ_DEPTH];	/* Current allotment per slot */
-	unsigned short	hash[SFQ_DEPTH];	/* Hash value indexed by slots */
-	struct sk_buff_head	qs[SFQ_DEPTH];		/* Slot queue */
-	struct sfq_head	dep[SFQ_DEPTH*2];	/* Linked list of slots, indexed by depth */
+	struct sfq_slot	slots[SFQ_SLOTS];
+	struct sfq_head	dep[SFQ_DEPTH];	/* Linked list of slots, indexed by depth */
 };
 
+/*
+ * sfq_head are either in a sfq_slot or in dep[] array
+ */
+static inline struct sfq_head *get_head(struct sfq_sched_data *q, sfq_index val)
+{
+	if (val < SFQ_SLOTS)
+		return &q->slots[val].anchor;
+	return &q->dep[val - SFQ_SLOTS];
+}
+
 static __inline__ unsigned sfq_fold_hash(struct sfq_sched_data *q, u32 h, u32 h1)
 {
 	return jhash_2words(h, h1, q->perturbation) & (SFQ_HASH_DIVISOR - 1);
@@ -200,30 +222,37 @@ static unsigned int sfq_classify(struct sk_buff *skb, struct Qdisc *sch,
 	return 0;
 }
 
+/*
+ * x : slot number [0 .. SFQ_SLOTS - 1]
+ */
 static inline void sfq_link(struct sfq_sched_data *q, sfq_index x)
 {
 	sfq_index p, n;
-	int d = q->qs[x].qlen + SFQ_DEPTH;
+	int qlen = q->slots[x].qlen;
 
-	p = d;
-	n = q->dep[d].next;
-	q->dep[x].next = n;
-	q->dep[x].prev = p;
-	q->dep[p].next = q->dep[n].prev = x;
+	p = qlen + SFQ_SLOTS;
+	n = q->dep[qlen].next;
+
+	q->slots[x].anchor.next = n;
+	q->slots[x].anchor.prev = p;
+
+	q->dep[qlen].next = x;		/* get_head(q, p)->next = x */
+	get_head(q, n)->prev = x;
 }
 
 static inline void sfq_dec(struct sfq_sched_data *q, sfq_index x)
 {
 	sfq_index p, n;
+	int d;
 
-	n = q->dep[x].next;
-	p = q->dep[x].prev;
-	q->dep[p].next = n;
-	q->dep[n].prev = p;
+	n = q->slots[x].anchor.next;
+	p = q->slots[x].anchor.prev;
+	get_head(q, p)->next = n;
+	get_head(q, n)->prev = p;
 
-	if (n == p && q->max_depth == q->qs[x].qlen + 1)
+	d = q->slots[x].qlen--;
+	if (n == p && q->max_depth == d)
 		q->max_depth--;
-
 	sfq_link(q, x);
 }
 
@@ -232,34 +261,36 @@ static inline void sfq_inc(struct sfq_sched_data *q, sfq_index x)
 	sfq_index p, n;
 	int d;
 
-	n = q->dep[x].next;
-	p = q->dep[x].prev;
-	q->dep[p].next = n;
-	q->dep[n].prev = p;
-	d = q->qs[x].qlen;
+	n = q->slots[x].anchor.next;
+	p = q->slots[x].anchor.prev;
+	get_head(q, p)->next = n;
+	get_head(q, n)->prev = p;
+
+	d = ++q->slots[x].qlen;
 	if (q->max_depth < d)
 		q->max_depth = d;
-
 	sfq_link(q, x);
 }
 
 static unsigned int sfq_drop(struct Qdisc *sch)
 {
 	struct sfq_sched_data *q = qdisc_priv(sch);
-	sfq_index d = q->max_depth;
+	sfq_index x, d = q->max_depth;
 	struct sk_buff *skb;
 	unsigned int len;
+	struct sfq_slot *slot;
 
-	/* Queue is full! Find the longest slot and
-	   drop a packet from it */
-
+	/* Queue is full! Find the longest slot and drop tail packet from it */
 	if (d > 1) {
-		sfq_index x = q->dep[d + SFQ_DEPTH].next;
-		skb = q->qs[x].prev;
+		x = q->dep[d].next;
+		slot = &q->slots[x];
+drop:
+		skb = slot->skblist_prev;
+		slot->skblist_prev = skb->prev;
 		len = qdisc_pkt_len(skb);
-		__skb_unlink(skb, &q->qs[x]);
-		kfree_skb(skb);
 		sfq_dec(q, x);
+		skb->next = skb->prev = NULL;
+		kfree_skb(skb);
 		sch->q.qlen--;
 		sch->qstats.drops++;
 		sch->qstats.backlog -= len;
@@ -268,19 +299,11 @@ static unsigned int sfq_drop(struct Qdisc *sch)
 
 	if (d == 1) {
 		/* It is difficult to believe, but ALL THE SLOTS HAVE LENGTH 1. */
-		d = q->next[q->tail];
-		q->next[q->tail] = q->next[d];
-		q->allot[q->next[d]] += q->quantum;
-		skb = q->qs[d].prev;
-		len = qdisc_pkt_len(skb);
-		__skb_unlink(skb, &q->qs[d]);
-		kfree_skb(skb);
-		sfq_dec(q, d);
-		sch->q.qlen--;
-		q->ht[q->hash[d]] = SFQ_DEPTH;
-		sch->qstats.drops++;
-		sch->qstats.backlog -= len;
-		return len;
+		x = q->tail->next;
+		slot = &q->slots[x];
+		q->tail->next = slot->next;
+		q->ht[slot->hash] = EMPTY_SLOT;
+		goto drop;
 	}
 
 	return 0;
@@ -292,6 +315,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	struct sfq_sched_data *q = qdisc_priv(sch);
 	unsigned int hash;
 	sfq_index x;
+	struct sfq_slot *slot;
 	int uninitialized_var(ret);
 
 	hash = sfq_classify(skb, sch, &ret);
@@ -304,31 +328,36 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	hash--;
 
 	x = q->ht[hash];
-	if (x == SFQ_DEPTH) {
-		q->ht[hash] = x = q->dep[SFQ_DEPTH].next;
-		q->hash[x] = hash;
+	slot = &q->slots[x];
+	if (x == EMPTY_SLOT) {
+		x = q->dep[0].next; /* get a free slot */
+		q->ht[hash] = x;
+		slot = &q->slots[x];
+		slot->hash = hash;
+		slot->skblist_next = slot->skblist_prev = (struct sk_buff *)slot;
 	}
 
-	/* If selected queue has length q->limit, this means that
-	 * all another queues are empty and that we do simple tail drop,
+	/* If selected queue has length q->limit, do simple tail drop,
 	 * i.e. drop _this_ packet.
 	 */
-	if (q->qs[x].qlen >= q->limit)
+	if (slot->qlen >= q->limit)
 		return qdisc_drop(skb, sch);
 
 	sch->qstats.backlog += qdisc_pkt_len(skb);
-	__skb_queue_tail(&q->qs[x], skb);
+	skb->prev = slot->skblist_prev;
+	skb->next = (struct sk_buff *)slot;
+	slot->skblist_prev->next = skb;
+	slot->skblist_prev = skb;
 	sfq_inc(q, x);
-	if (q->qs[x].qlen == 1) {		/* The flow is new */
-		if (q->tail == SFQ_DEPTH) {	/* It is the first flow */
-			q->tail = x;
-			q->next[x] = x;
-			q->allot[x] = q->quantum;
+	if (slot->qlen == 1) {		/* The flow is new */
+		if (q->tail == NULL) {	/* It is the first flow */
+			slot->next = x;
 		} else {
-			q->next[x] = q->next[q->tail];
-			q->next[q->tail] = x;
-			q->tail = x;
+			slot->next = q->tail->next;
+			q->tail->next = x;
 		}
+		q->tail = slot;
+		slot->allot = q->quantum;
 	}
 	if (++sch->q.qlen <= q->limit) {
 		sch->bstats.bytes += qdisc_pkt_len(skb);
@@ -344,14 +373,12 @@ static struct sk_buff *
 sfq_peek(struct Qdisc *sch)
 {
 	struct sfq_sched_data *q = qdisc_priv(sch);
-	sfq_index a;
 
 	/* No active slots */
-	if (q->tail == SFQ_DEPTH)
+	if (q->tail == NULL)
 		return NULL;
 
-	a = q->next[q->tail];
-	return skb_peek(&q->qs[a]);
+	return q->slots[q->tail->next].skblist_next;
 }
 
 static struct sk_buff *
@@ -359,34 +386,35 @@ sfq_dequeue(struct Qdisc *sch)
 {
 	struct sfq_sched_data *q = qdisc_priv(sch);
 	struct sk_buff *skb;
-	sfq_index a, old_a;
+	sfq_index a, next_a;
+	struct sfq_slot *slot;
 
 	/* No active slots */
-	if (q->tail == SFQ_DEPTH)
+	if (q->tail == NULL)
 		return NULL;
 
-	a = old_a = q->next[q->tail];
-
+	a = q->tail->next;
+	slot = &q->slots[a];
 	/* Grab packet */
-	skb = __skb_dequeue(&q->qs[a]);
+	skb = slot->skblist_next;
+	slot->skblist_next = skb->next;
+	skb->next = skb->prev = NULL;
 	sfq_dec(q, a);
 	sch->q.qlen--;
 	sch->qstats.backlog -= qdisc_pkt_len(skb);
 
-	/* Is the slot empty? */
-	if (q->qs[a].qlen == 0) {
-		q->ht[q->hash[a]] = SFQ_DEPTH;
-		a = q->next[a];
-		if (a == old_a) {
-			q->tail = SFQ_DEPTH;
+	/* Is the slot now empty? */
+	if (slot->qlen == 0) {
+		q->ht[slot->hash] = EMPTY_SLOT;
+		next_a = slot->next;
+		if (a == next_a) {
+			q->tail = NULL; /* no more active slots */
 			return skb;
 		}
-		q->next[q->tail] = a;
-		q->allot[a] += q->quantum;
-	} else if ((q->allot[a] -= qdisc_pkt_len(skb)) <= 0) {
-		q->tail = a;
-		a = q->next[a];
-		q->allot[a] += q->quantum;
+		q->tail->next = next_a;
+	} else if ((slot->allot -= qdisc_pkt_len(skb)) <= 0) {
+		q->tail = slot;
+		slot->allot += q->quantum;
 	}
 	return skb;
 }
@@ -450,17 +478,16 @@ static int sfq_init(struct Qdisc *sch, struct nlattr *opt)
 	init_timer_deferrable(&q->perturb_timer);
 
 	for (i = 0; i < SFQ_HASH_DIVISOR; i++)
-		q->ht[i] = SFQ_DEPTH;
+		q->ht[i] = EMPTY_SLOT;
 
 	for (i = 0; i < SFQ_DEPTH; i++) {
-		skb_queue_head_init(&q->qs[i]);
-		q->dep[i + SFQ_DEPTH].next = i + SFQ_DEPTH;
-		q->dep[i + SFQ_DEPTH].prev = i + SFQ_DEPTH;
+		q->dep[i].next = i + SFQ_SLOTS;
+		q->dep[i].prev = i + SFQ_SLOTS;
 	}
 
 	q->limit = SFQ_DEPTH - 1;
 	q->max_depth = 0;
-	q->tail = SFQ_DEPTH;
+	q->tail = NULL;
 	if (opt == NULL) {
 		q->quantum = psched_mtu(qdisc_dev(sch));
 		q->perturb_period = 0;
@@ -471,7 +498,7 @@ static int sfq_init(struct Qdisc *sch, struct nlattr *opt)
 			return err;
 	}
 
-	for (i = 0; i < SFQ_DEPTH; i++)
+	for (i = 0; i < SFQ_SLOTS; i++)
 		sfq_link(q, i);
 	return 0;
 }
@@ -547,9 +574,9 @@ static int sfq_dump_class_stats(struct Qdisc *sch, unsigned long cl,
 				struct gnet_dump *d)
 {
 	struct sfq_sched_data *q = qdisc_priv(sch);
-	sfq_index idx = q->ht[cl-1];
-	struct gnet_stats_queue qs = { .qlen = q->qs[idx].qlen };
-	struct tc_sfq_xstats xstats = { .allot = q->allot[idx] };
+	const struct sfq_slot *slot = &q->slots[q->ht[cl - 1]];
+	struct gnet_stats_queue qs = { .qlen = slot->qlen };
+	struct tc_sfq_xstats xstats = { .allot = slot->allot };
 
 	if (gnet_stats_copy_queue(d, &qs) < 0)
 		return -1;
@@ -565,7 +592,7 @@ static void sfq_walk(struct Qdisc *sch, struct qdisc_walker *arg)
 		return;
 
 	for (i = 0; i < SFQ_HASH_DIVISOR; i++) {
-		if (q->ht[i] == SFQ_DEPTH ||
+		if (q->ht[i] == EMPTY_SLOT ||
 		    arg->count < arg->skip) {
 			arg->count++;
 			continue;


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