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authorEric Dumazet <edumazet@google.com>2016-09-22 08:58:55 -0700
committerDavid S. Miller <davem@davemloft.net>2016-09-23 07:19:06 -0400
commitfefa569a9d4bc4b7758c0fddd75bb0382c95da77 (patch)
tree5a706c52323a8f056a8ac5cfa045f3b4630aba67 /net/sched/sch_fq.c
parentsfc: check async completer is !NULL before calling (diff)
downloadlinux-dev-fefa569a9d4bc4b7758c0fddd75bb0382c95da77.tar.xz
linux-dev-fefa569a9d4bc4b7758c0fddd75bb0382c95da77.zip
net_sched: sch_fq: account for schedule/timers drifts
It looks like the following patch can make FQ very precise, even in VM or stressed hosts. It matters at high pacing rates. We take into account the difference between the time that was programmed when last packet was sent, and current time (a drift of tens of usecs is often observed) Add an EWMA of the unthrottle latency to help diagnostics. This latency is the difference between current time and oldest packet in delayed RB-tree. This accounts for the high resolution timer latency, but can be different under stress, as fq_check_throttled() can be opportunistically be called from a dequeue() called after an enqueue() for a different flow. Tested: // Start a 10Gbit flow $ netperf --google-pacing-rate 1250000000 -H lpaa24 -l 10000 -- -K bbr & Before patch : $ sar -n DEV 10 5 | grep eth0 | grep Average Average: eth0 17106.04 756876.84 1102.75 1119049.02 0.00 0.00 0.52 After patch : $ sar -n DEV 10 5 | grep eth0 | grep Average Average: eth0 17867.00 800245.90 1151.77 1183172.12 0.00 0.00 0.52 A new iproute2 tc can output the 'unthrottle latency' : $ tc -s qd sh dev eth0 | grep latency 0 gc, 0 highprio, 32490767 throttled, 2382 ns latency Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/sched/sch_fq.c')
-rw-r--r--net/sched/sch_fq.c21
1 files changed, 18 insertions, 3 deletions
diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 5dd929cc1423..18e752439f6f 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -86,6 +86,7 @@ struct fq_sched_data {
struct rb_root delayed; /* for rate limited flows */
u64 time_next_delayed_flow;
+ unsigned long unthrottle_latency_ns;
struct fq_flow internal; /* for non classified or high prio packets */
u32 quantum;
@@ -408,11 +409,19 @@ static int fq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
static void fq_check_throttled(struct fq_sched_data *q, u64 now)
{
+ unsigned long sample;
struct rb_node *p;
if (q->time_next_delayed_flow > now)
return;
+ /* Update unthrottle latency EWMA.
+ * This is cheap and can help diagnosing timer/latency problems.
+ */
+ sample = (unsigned long)(now - q->time_next_delayed_flow);
+ q->unthrottle_latency_ns -= q->unthrottle_latency_ns >> 3;
+ q->unthrottle_latency_ns += sample >> 3;
+
q->time_next_delayed_flow = ~0ULL;
while ((p = rb_first(&q->delayed)) != NULL) {
struct fq_flow *f = container_of(p, struct fq_flow, rate_node);
@@ -515,7 +524,12 @@ begin:
len = NSEC_PER_SEC;
q->stat_pkts_too_long++;
}
-
+ /* Account for schedule/timers drifts.
+ * f->time_next_packet was set when prior packet was sent,
+ * and current time (@now) can be too late by tens of us.
+ */
+ if (f->time_next_packet)
+ len -= min(len/2, now - f->time_next_packet);
f->time_next_packet = now + len;
}
out:
@@ -787,6 +801,7 @@ static int fq_init(struct Qdisc *sch, struct nlattr *opt)
q->initial_quantum = 10 * psched_mtu(qdisc_dev(sch));
q->flow_refill_delay = msecs_to_jiffies(40);
q->flow_max_rate = ~0U;
+ q->time_next_delayed_flow = ~0ULL;
q->rate_enable = 1;
q->new_flows.first = NULL;
q->old_flows.first = NULL;
@@ -854,8 +869,8 @@ static int fq_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
st.flows = q->flows;
st.inactive_flows = q->inactive_flows;
st.throttled_flows = q->throttled_flows;
- st.pad = 0;
-
+ st.unthrottle_latency_ns = min_t(unsigned long,
+ q->unthrottle_latency_ns, ~0U);
sch_tree_unlock(sch);
return gnet_stats_copy_app(d, &st, sizeof(st));