netdev - [PATCH net-next 2/2] net: dev: introduce support for sch BYPASS for lockless qdisc

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Date:   Thu, 21 Mar 2019 11:14:19 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        John Fastabend <john.fastabend@...il.com>,
        Ivan Vecera <ivecera@...hat.com>
Subject: [PATCH net-next 2/2] net: dev: introduce support for sch BYPASS for lockless qdisc

With commit c5ad119fb6c0 ("net: sched: pfifo_fast use skb_array")
pfifo_fast no longer benefit from the TCQ_F_CAN_BYPASS optimization.
Due to retpolines the cost of the enqueue()/dequeue() pair has become
relevant and we observe measurable regression for the uncontended
scenario when the packet-rate is below line rate.

After commit 46b1c18f9deb ("net: sched: put back q.qlen into a
single location") we can check for empty qdisc with a reasonably
fast operation even for nolock qdiscs.

This change extends TCQ_F_CAN_BYPASS support to nolock qdisc.
The new chunk of code mirrors closely the existing one for traditional
qdisc, leveraging a newly introduced helper to read atomically the
qdisc length.

Tested with pktgen in queue xmit mode, with pfifo_fast, a MQ
device, and MQ root qdisc:

threads         vanilla         patched
                kpps            kpps
1               2465            2889
2               4304            5188
4               7898            9589

Same as above, but with a single queue device:

threads         vanilla         patched
                kpps            kpps
1               2556            2827
2               2900            2900
4               5000            5000
8               4700            4700

No mesaurable changes in the contended scenarios, and more 10%
improvement in the uncontended ones.

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
Tested-by: Ivan Vecera <ivecera@...hat.com>
---
 net/core/dev.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 2b67f2aa59dd..9a6d3617f03a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3468,6 +3468,15 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 		if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
 			__qdisc_drop(skb, &to_free);
 			rc = NET_XMIT_DROP;
+		} else if ((q->flags & TCQ_F_CAN_BYPASS) && !q->not_empty &&
+			   qdisc_run_begin(q)) {
+			qdisc_bstats_cpu_update(q, skb);
+
+			if (sch_direct_xmit(skb, q, dev, txq, NULL, true))
+				__qdisc_run(q);
+
+			qdisc_run_end(q);
+			rc = NET_XMIT_SUCCESS;
 		} else {
 			rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
 			qdisc_run(q);
-- 
2.20.1