Subject: Re: Quota in __qdisc_run() (was: qdisc: validate skb without holding lock)

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 7 Oct 2014 15:30:50 +0200

On Tue, 07 Oct 2014 05:47:18 -0700
Eric Dumazet <[email-masked]> wrote:

> On Tue, 2014-10-07 at 09:34 +0200, Jesper Dangaard Brouer wrote:
> > On Fri, 03 Oct 2014 16:30:44 -0700 Eric Dumazet <[email-masked]> wrote:
> > 
> > > Another problem we need to address is the quota in __qdisc_run()
> > > is no longer meaningfull, if each qdisc_restart() can pump many packets.
> > 
> > I fully agree. My earlier "magic" packet limit was covering/pampering
> > over this issue.
> 
> Although quota was multiplied by 7 or 8 in worst case ?

Yes, exactly not a very elegant solution ;-)

> > > An idea would be to use the bstats (or cpu_qstats if applicable)
> > 
> > Please elaborate some more, as I don't completely follow (feel free to
> > show with a patch ;-)).
> > 
> 
> I was hoping John could finish the percpu stats before I do that.
> 
> Problem with q->bstats.packets is that TSO packets with 45 MSS add 45 to
> this counter.
> 
> Using a time quota would be better, but : jiffies is too big, and
> local_clock() might be too expensive.

Hannes hacked up this patch for me... (didn't finish testing)

The basic idea is we want keep/restore the quota fairness between
qdisc's , that we sort of broke with commit 5772e9a346 ("qdisc: bulk
dequeue support for qdiscs with TCQ_F_ONETXQUEUE").

We choose not to account for the number of packets inside the TSO/GSO
packets ("skb_gso_segs").  As the previous fairness also had this "defect".

You might view this as a short term solution, until you can fix it with
your q->bstats.packets or time quota?

[RFC PATCH] net_sched: restore quota limits after bulk dequeue

--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -57,17 +57,19 @@ static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
 
 static void try_bulk_dequeue_skb(struct Qdisc *q,
                                 struct sk_buff *skb,
-                                const struct netdev_queue *txq)
+                                const struct netdev_queue *txq,
+                                int *quota)
 {
        int bytelimit = qdisc_avail_bulklimit(txq) - skb->len;
 
-       while (bytelimit > 0) {
+       while (bytelimit > 0 && *quota > 0) {
                struct sk_buff *nskb = q->dequeue(q);
 
                if (!nskb)
                        break;
 
                bytelimit -= nskb->len; /* covers GSO len */
+               --*quota;
                skb->next = nskb;
                skb = nskb;
        }
@@ -77,7 +79,7 @@ static void try_bulk_dequeue_skb(struct Qdisc *q,
 /* Note that dequeue_skb can possibly return a SKB list (via skb->next).
  * A requeued skb (via q->gso_skb) can also be a SKB list.
  */
-static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate)
+static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate, int *quota)
 {
        struct sk_buff *skb = q->gso_skb;
        const struct netdev_queue *txq = q->dev_queue;
@@ -87,18 +89,25 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate)
                /* check the reason of requeuing without tx lock first */
                txq = skb_get_tx_queue(txq->dev, skb);
                if (!netif_xmit_frozen_or_stopped(txq)) {
+                       struct sk_buff *iskb = skb;
+
                        q->gso_skb = NULL;
                        q->q.qlen--;
-               } else
+                       do
+                               --*quota;
+                       while ((iskb = skb->next));
+               } else {
                        skb = NULL;
+               }
                /* skb in gso_skb were already validated */
                *validate = false;
        } else {
                if (!(q->flags & TCQ_F_ONETXQUEUE) ||
                    !netif_xmit_frozen_or_stopped(txq)) {
                        skb = q->dequeue(q);
+                       --*quota;
                        if (skb && qdisc_may_bulk(q))
-                               try_bulk_dequeue_skb(q, skb, txq);
+                               try_bulk_dequeue_skb(q, skb, txq, quota);
                }
        }
        return skb;
@@ -204,7 +213,7 @@ int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
  *                             >0 - queue is not empty.
  *
  */
-static inline int qdisc_restart(struct Qdisc *q)
+static inline int qdisc_restart(struct Qdisc *q, int *quota)
 {
        struct netdev_queue *txq;
        struct net_device *dev;
@@ -213,7 +222,7 @@ static inline int qdisc_restart(struct Qdisc *q)
        bool validate;
 
        /* Dequeue packet */
-       skb = dequeue_skb(q, &validate);
+       skb = dequeue_skb(q, &validate, quota);
        if (unlikely(!skb))
                return 0;
 
@@ -230,13 +239,13 @@ void __qdisc_run(struct Qdisc *q)
 {
        int quota = weight_p;
 
-       while (qdisc_restart(q)) {
+       while (qdisc_restart(q, &quota)) {
                /*
                 * Ordered by possible occurrence: Postpone processing if
                 * 1. we've exceeded packet quota
                 * 2. another process needs the CPU;
                 */
-               if (--quota <= 0 || need_resched()) {
+               if (quota <= 0 || need_resched()) {
                        __netif_schedule(q);
                        break;
                }