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net: sched: Remove Qdisc::running sequence counter

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The Qdisc::running sequence counter has two uses:

  1. Reliably reading qdisc's tc statistics while the qdisc is running
     (a seqcount read/retry loop at gnet_stats_add_basic()).

  2. As a flag, indicating whether the qdisc in question is running
     (without any retry loops).

For the first usage, the Qdisc::running sequence counter write section,
qdisc_run_begin() => qdisc_run_end(), covers a much wider area than what
is actually needed: the raw qdisc's bstats update. A u64_stats sync
point was thus introduced (in previous commits) inside the bstats
structure itself. A local u64_stats write section is then started and
stopped for the bstats updates.

Use that u64_stats sync point mechanism for the bstats read/retry loop
at gnet_stats_add_basic().

For the second qdisc->running usage, a __QDISC_STATE_RUNNING bit flag,
accessed with atomic bitops, is sufficient. Using a bit flag instead of
a sequence counter at qdisc_run_begin/end() and qdisc_is_running() leads
to the SMP barriers implicitly added through raw_read_seqcount() and
write_seqcount_begin/end() getting removed. All call sites have been
surveyed though, and no required ordering was identified.

Now that the qdisc->running sequence counter is no longer used, remove
it.

Note, using u64_stats implies no sequence counter protection for 64-bit
architectures. This can lead to the qdisc tc statistics "packets" vs.
"bytes" values getting out of sync on rare occasions. The individual
values will still be valid.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Ahmed S. Darwish 2021-10-16 10:49:10 +02:00 • committed by David S. Miller
parent 50dc9a8572
commit 29cbcd8582
21 changed files with 102 additions and 134 deletions
Download patch file Download diff file Expand all files Collapse all files

16
net/sched/sch_api.c
View file

@ -943,8 +943,7 @@ static int tc_fill_qdisc(struct sk_buff *skb, struct Qdisc *q, u32 clid,
cpu_qstats = q->cpu_qstats;
}
if (gnet_stats_copy_basic(qdisc_root_sleeping_running(q),
&d, cpu_bstats, &q->bstats) < 0 ||
if (gnet_stats_copy_basic(&d, cpu_bstats, &q->bstats, true) < 0 ||
gnet_stats_copy_rate_est(&d, &q->rate_est) < 0 ||
gnet_stats_copy_queue(&d, cpu_qstats, &q->qstats, qlen) < 0)
goto nla_put_failure;
@ -1265,26 +1264,17 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
rcu_assign_pointer(sch->stab, stab);
}
if (tca[TCA_RATE]) {
seqcount_t *running;
err = -EOPNOTSUPP;
if (sch->flags & TCQ_F_MQROOT) {
NL_SET_ERR_MSG(extack, "Cannot attach rate estimator to a multi-queue root qdisc");
goto err_out4;
}
if (sch->parent != TC_H_ROOT &&
!(sch->flags & TCQ_F_INGRESS) &&
(!p || !(p->flags & TCQ_F_MQROOT)))
running = qdisc_root_sleeping_running(sch);
else
running = &sch->running;
err = gen_new_estimator(&sch->bstats,
sch->cpu_bstats,
&sch->rate_est,
NULL,
running,
true,
tca[TCA_RATE]);
if (err) {
NL_SET_ERR_MSG(extack, "Failed to generate new estimator");
@ -1360,7 +1350,7 @@ static int qdisc_change(struct Qdisc *sch, struct nlattr **tca,
sch->cpu_bstats,
&sch->rate_est,
NULL,
qdisc_root_sleeping_running(sch),
true,
tca[TCA_RATE]);
}
out: