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authorYunsheng Lin <linyunsheng@huawei.com>2021-05-14 11:16:59 +0800
committerDavid S. Miller <davem@davemloft.net>2021-05-14 15:05:46 -0700
commita90c57f2cedd52a511f739fb55e6244e22e1a2fb (patch)
treea6ee296ff4caabf659dc004e4bbf2e81467dd3cb /net/sched/sch_generic.c
parent974271e5ed45cfe4daddbeb16224a2156918530e (diff)
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net: sched: fix packet stuck problem for lockless qdisc
Lockless qdisc has below concurrent problem: cpu0 cpu1 . . q->enqueue . . . qdisc_run_begin() . . . dequeue_skb() . . . sch_direct_xmit() . . . . q->enqueue . qdisc_run_begin() . return and do nothing . . qdisc_run_end() . cpu1 enqueue a skb without calling __qdisc_run() because cpu0 has not released the lock yet and spin_trylock() return false for cpu1 in qdisc_run_begin(), and cpu0 do not see the skb enqueued by cpu1 when calling dequeue_skb() because cpu1 may enqueue the skb after cpu0 calling dequeue_skb() and before cpu0 calling qdisc_run_end(). Lockless qdisc has below another concurrent problem when tx_action is involved: cpu0(serving tx_action) cpu1 cpu2 . . . . q->enqueue . . qdisc_run_begin() . . dequeue_skb() . . . q->enqueue . . . . sch_direct_xmit() . . . qdisc_run_begin() . . return and do nothing . . . clear __QDISC_STATE_SCHED . . qdisc_run_begin() . . return and do nothing . . . . . . qdisc_run_end() . This patch fixes the above data race by: 1. If the first spin_trylock() return false and STATE_MISSED is not set, set STATE_MISSED and retry another spin_trylock() in case other CPU may not see STATE_MISSED after it releases the lock. 2. reschedule if STATE_MISSED is set after the lock is released at the end of qdisc_run_end(). For tx_action case, STATE_MISSED is also set when cpu1 is at the end if qdisc_run_end(), so tx_action will be rescheduled again to dequeue the skb enqueued by cpu2. Clear STATE_MISSED before retrying a dequeuing when dequeuing returns NULL in order to reduce the overhead of the second spin_trylock() and __netif_schedule() calling. Also clear the STATE_MISSED before calling __netif_schedule() at the end of qdisc_run_end() to avoid doing another round of dequeuing in the pfifo_fast_dequeue(). The performance impact of this patch, tested using pktgen and dummy netdev with pfifo_fast qdisc attached: threads without+this_patch with+this_patch delta 1 2.61Mpps 2.60Mpps -0.3% 2 3.97Mpps 3.82Mpps -3.7% 4 5.62Mpps 5.59Mpps -0.5% 8 2.78Mpps 2.77Mpps -0.3% 16 2.22Mpps 2.22Mpps -0.0% Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking") Acked-by: Jakub Kicinski <kuba@kernel.org> Tested-by: Juergen Gross <jgross@suse.com> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/sched/sch_generic.c')
-rw-r--r--net/sched/sch_generic.c19
1 files changed, 19 insertions, 0 deletions
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 44991ea726fc..795d986e7030 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -640,8 +640,10 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
{
struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
struct sk_buff *skb = NULL;
+ bool need_retry = true;
int band;
+retry:
for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
struct skb_array *q = band2list(priv, band);
@@ -652,6 +654,23 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
}
if (likely(skb)) {
qdisc_update_stats_at_dequeue(qdisc, skb);
+ } else if (need_retry &&
+ test_bit(__QDISC_STATE_MISSED, &qdisc->state)) {
+ /* Delay clearing the STATE_MISSED here to reduce
+ * the overhead of the second spin_trylock() in
+ * qdisc_run_begin() and __netif_schedule() calling
+ * in qdisc_run_end().
+ */
+ clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
+
+ /* Make sure dequeuing happens after clearing
+ * STATE_MISSED.
+ */
+ smp_mb__after_atomic();
+
+ need_retry = false;
+
+ goto retry;
} else {
WRITE_ONCE(qdisc->empty, true);
}