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Date: Sat, 02 Mar 2019 14:11:43 -0800 (PST) From: David Miller <davem@...emloft.net> To: edumazet@...gle.com Cc: netdev@...r.kernel.org, eric.dumazet@...il.com, john.fastabend@...il.com, jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us Subject: Re: [PATCH net] net: sched: put back q.qlen into a single location From: Eric Dumazet <edumazet@...gle.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@...gle.com> Applied and queued up for -stable, thanks Eric.
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