Re: [PATCH net] net: sched: put back q.qlen into a single location

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <edumazet@google.com>
Date: Thu, 28 Feb 2019 12:55:43 -0800

> In the series fc8b81a5981f ("Merge branch 'lockless-qdisc-series'")
> John made the assumption that the data path had no need to read
> the qdisc qlen (number of packets in the qdisc).
> 
> It is true when pfifo_fast is used as the root qdisc, or as direct MQ/MQPRIO
> children.
> 
> But pfifo_fast can be used as leaf in class full qdiscs, and existing
> logic needs to access the child qlen in an efficient way.
> 
> HTB breaks badly, since it uses cl->leaf.q->q.qlen in :
>   htb_activate() -> WARN_ON()
>   htb_dequeue_tree() to decide if a class can be htb_deactivated
>   when it has no more packets.
> 
> HFSC, DRR, CBQ, QFQ have similar issues, and some calls to
> qdisc_tree_reduce_backlog() also read q.qlen directly.
 ...
> This patch adds atomic_qlen (in the same location than qlen)
> and renames the following helpers, since we want to express
> they can be used without qdisc lock, and that qlen is no longer percpu.
> 
> - qdisc_qstats_cpu_qlen_dec -> qdisc_qstats_atomic_qlen_dec()
> - qdisc_qstats_cpu_qlen_inc -> qdisc_qstats_atomic_qlen_inc()
> 
> Later (net-next) we might revert this patch by tracking all these
> qlen uses and replace them by a more efficient method (not having
> to access a precise qlen, but an empty/non_empty status that might
> be less expensive to maintain/track).
> 
> Another possibility is to have a legacy pfifo_fast version that would
> be used when used a a child qdisc, since the parent qdisc needs
> a spinlock anyway. But then, future lockless qdiscs would also
> have the same problem.
> 
> Fixes: 7e66016f2c65 ("net: sched: helpers to sum qlen and qlen for per cpu logic")
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Applied and queued up for -stable, thanks Eric.