[net-next,v3,1/2] net: sched: add empty status flag for NOLOCK qdisc

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>
---
 include/net/sch_generic.h | 11 +++++++++++
 net/sched/sch_generic.c   |  3 +++
 2 files changed, 14 insertions(+)

On 03/22/2019 08:01 AM, Paolo Abeni wrote:
> 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.
> 

Reviewed-by: Eric Dumazet <edumazet@google.com>

On 22. 03. 19 16:01, Paolo Abeni wrote:
> 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>
> ---
>   include/net/sch_generic.h | 11 +++++++++++
>   net/sched/sch_generic.c   |  3 +++
>   2 files changed, 14 insertions(+)
> 
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index 31284c078d06..e227475e78ca 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -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;
>   	}
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index a117d9260558..81356ef38d1d 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -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);
>   
> 
Reviewed-by: Ivan Vecera <ivecera@redhat.com>