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net: sched: add empty status flag for NOLOCK qdisc

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The queue is marked not empty after acquiring the seqlock,
and it's up to the NOLOCK qdisc clearing such flag on dequeue.
Since the empty status lays on the same cache-line of the
seqlock, it's always hot on cache during the updates.

This makes the empty flag update a little bit loosy. Given
the lack of synchronization between enqueue and dequeue, this
is unavoidable.

v2 -> v3:
 - qdisc_is_empty() has a const argument (Eric)

v1 -> v2:
 - use really an 'empty' flag instead of 'not_empty', as
   suggested by Eric

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Paolo Abeni 2019-03-22 16:01:55 +01:00 committed by David S. Miller
parent 576fd2f7ca
commit 28cff537ef
2 changed files with 14 additions and 0 deletions
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11
include/net/sch_generic.h
View File

@ -113,6 +113,9 @@ struct Qdisc {
spinlock_t busylock ____cacheline_aligned_in_smp;
spinlock_t seqlock;
/* for NOLOCK qdisc, true if there are no enqueued skbs */
bool empty;
struct rcu_head rcu;
};
@ -143,11 +146,19 @@ static inline bool qdisc_is_running(struct Qdisc *qdisc)
return (raw_read_seqcount(&qdisc->running) & 1) ? true : false;
}
static inline bool qdisc_is_empty(const struct Qdisc *qdisc)
{
if (qdisc->flags & TCQ_F_NOLOCK)
return qdisc->empty;
return !qdisc->q.qlen;
}
static inline bool qdisc_run_begin(struct Qdisc *qdisc)
{
if (qdisc->flags & TCQ_F_NOLOCK) {
if (!spin_trylock(&qdisc->seqlock))
return false;
qdisc->empty = false;
} else if (qdisc_is_running(qdisc)) {
return false;
}

3
net/sched/sch_generic.c
View File

@ -671,6 +671,8 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
qdisc_qstats_cpu_backlog_dec(qdisc, skb);
qdisc_bstats_cpu_update(qdisc, skb);
qdisc_qstats_atomic_qlen_dec(qdisc);
} else {
qdisc->empty = true;
}
return skb;
@ -880,6 +882,7 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
sch->enqueue = ops->enqueue;
sch->dequeue = ops->dequeue;
sch->dev_queue = dev_queue;
sch->empty = true;
dev_hold(dev);
refcount_set(&sch->refcnt, 1);