net: sched: do not acquire qdisc spinlock in qdisc/class stats dump

Large tc dumps (tc -s {qdisc|class} sh dev ethX) done by Google BwE host
agent [1] are problematic at scale :

For each qdisc/class found in the dump, we currently lock the root qdisc
spinlock in order to get stats. Sampling stats every 5 seconds from
thousands of HTB classes is a challenge when the root qdisc spinlock is
under high pressure. Not only the dumps take time, they also slow
down the fast path (queue/dequeue packets) by 10 % to 20 % in some cases.

An audit of existing qdiscs showed that sch_fq_codel is the only qdisc
that might need the qdisc lock in fq_codel_dump_stats() and
fq_codel_dump_class_stats()

In v2 of this patch, I now use the Qdisc running seqcount to provide
consistent reads of packets/bytes counters, regardless of 32/64 bit arches.

I also changed rate estimators to use the same infrastructure
so that they no longer need to lock root qdisc lock.

[1]
http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/43838.pdf

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Kevin Athey <kda@google.com>
Cc: Xiaotian Pei <xiaotian@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 5 deletions

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index d783d7cc3348..74813dd49053 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -1015,11 +1015,10 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
                 cur_time = psched_get_time();
 
                 if (tca[TCA_RATE]) {
-                        spinlock_t *lock = qdisc_root_sleeping_lock(sch);
-
                         err = gen_replace_estimator(&cl->bstats, NULL,
                                                     &cl->rate_est,
-                                                    lock,
+                                                    NULL,
+                                                    qdisc_root_sleeping_running(sch),
                                                     tca[TCA_RATE]);
                         if (err)
                                 return err;
@@ -1068,7 +1067,8 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 
         if (tca[TCA_RATE]) {
                 err = gen_new_estimator(&cl->bstats, NULL, &cl->rate_est,
-                                        qdisc_root_sleeping_lock(sch),
+                                        NULL,
+                                        qdisc_root_sleeping_running(sch),
                                         tca[TCA_RATE]);
                 if (err) {
                         kfree(cl);
@@ -1373,7 +1373,7 @@ hfsc_dump_class_stats(struct Qdisc *sch, unsigned long arg,
         xstats.work    = cl->cl_total;
         xstats.rtwork  = cl->cl_cumul;
 
-        if (gnet_stats_copy_basic(d, NULL, &cl->bstats) < 0 ||
+        if (gnet_stats_copy_basic(qdisc_root_sleeping_running(sch), d, NULL, &cl->bstats) < 0 ||
             gnet_stats_copy_rate_est(d, &cl->bstats, &cl->rate_est) < 0 ||
             gnet_stats_copy_queue(d, NULL, &cl->qstats, cl->qdisc->q.qlen) < 0)
                 return -1;