mac80211: Implement Airtime-based Queue Limit (AQL)

In order for the Fq_CoDel algorithm integrated in mac80211 layer to operate
effectively to control excessive queueing latency, the CoDel algorithm
requires an accurate measure of how long packets stays in the queue, AKA
sojourn time. The sojourn time measured at the mac80211 layer doesn't
include queueing latency in the lower layer (firmware/hardware) and CoDel
expects lower layer to have a short queue. However, most 802.11ac chipsets
offload tasks such TX aggregation to firmware or hardware, thus have a deep
lower layer queue.

Without a mechanism to control the lower layer queue size, packets only
stay in mac80211 layer transiently before being sent to firmware queue.
As a result, the sojourn time measured by CoDel in the mac80211 layer is
almost always lower than the CoDel latency target, hence CoDel does little
to control the latency, even when the lower layer queue causes excessive
latency.

The Byte Queue Limits (BQL) mechanism is commonly used to address the
similar issue with wired network interface. However, this method cannot be
applied directly to the wireless network interface. "Bytes" is not a
suitable measure of queue depth in the wireless network, as the data rate
can vary dramatically from station to station in the same network, from a
few Mbps to over Gbps.

This patch implements an Airtime-based Queue Limit (AQL) to make CoDel work
effectively with wireless drivers that utilized firmware/hardware
offloading. AQL allows each txq to release just enough packets to the lower
layer to form 1-2 large aggregations to keep hardware fully utilized and
retains the rest of the frames in mac80211 layer to be controlled by the
CoDel algorithm.

Signed-off-by: Kan Yan <kyan@google.com>
[ Toke: Keep API to set pending airtime internal, fix nits in commit msg ]
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20191119060610.76681-4-kyan@google.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 33 insertions, 10 deletions

diff --git a/net/mac80211/debugfs_sta.c b/net/mac80211/debugfs_sta.c
index c8ad20c28c43..0185e6e5e5d1 100644
--- a/net/mac80211/debugfs_sta.c
+++ b/net/mac80211/debugfs_sta.c
@@ -197,10 +197,12 @@ static ssize_t sta_airtime_read(struct file *file, char __user *userbuf,
 {
 	struct sta_info *sta = file->private_data;
 	struct ieee80211_local *local = sta->sdata->local;
-	size_t bufsz = 200;
+	size_t bufsz = 400;
 	char *buf = kzalloc(bufsz, GFP_KERNEL), *p = buf;
 	u64 rx_airtime = 0, tx_airtime = 0;
 	s64 deficit[IEEE80211_NUM_ACS];
+	u32 q_depth[IEEE80211_NUM_ACS];
+	u32 q_limit_l[IEEE80211_NUM_ACS], q_limit_h[IEEE80211_NUM_ACS];
 	ssize_t rv;
 	int ac;
 
@@ -212,19 +214,22 @@ static ssize_t sta_airtime_read(struct file *file, char __user *userbuf,
 		rx_airtime += sta->airtime[ac].rx_airtime;
 		tx_airtime += sta->airtime[ac].tx_airtime;
 		deficit[ac] = sta->airtime[ac].deficit;
+		q_limit_l[ac] = sta->airtime[ac].aql_limit_low;
+		q_limit_h[ac] = sta->airtime[ac].aql_limit_high;
 		spin_unlock_bh(&local->active_txq_lock[ac]);
+		q_depth[ac] = atomic_read(&sta->airtime[ac].aql_tx_pending);
 	}
 
 	p += scnprintf(p, bufsz + buf - p,
 		"RX: %llu us\nTX: %llu us\nWeight: %u\n"
-		"Deficit: VO: %lld us VI: %lld us BE: %lld us BK: %lld us\n",
-		rx_airtime,
-		tx_airtime,
-		sta->airtime_weight,
-		deficit[0],
-		deficit[1],
-		deficit[2],
-		deficit[3]);
+		"Deficit: VO: %lld us VI: %lld us BE: %lld us BK: %lld us\n"
+		"Q depth: VO: %u us VI: %u us BE: %u us BK: %u us\n"
+		"Q limit[low/high]: VO: %u/%u VI: %u/%u BE: %u/%u BK: %u/%u\n",
+		rx_airtime, tx_airtime, sta->airtime_weight,
+		deficit[0], deficit[1], deficit[2], deficit[3],
+		q_depth[0], q_depth[1], q_depth[2], q_depth[3],
+		q_limit_l[0], q_limit_h[0], q_limit_l[1], q_limit_h[1],
+		q_limit_l[2], q_limit_h[2], q_limit_l[3], q_limit_h[3]),
 
 	rv = simple_read_from_buffer(userbuf, count, ppos, buf, p - buf);
 	kfree(buf);
@@ -236,7 +241,25 @@ static ssize_t sta_airtime_write(struct file *file, const char __user *userbuf,
 {
 	struct sta_info *sta = file->private_data;
 	struct ieee80211_local *local = sta->sdata->local;
-	int ac;
+	u32 ac, q_limit_l, q_limit_h;
+	char _buf[100] = {}, *buf = _buf;
+
+	if (count > sizeof(_buf))
+		return -EINVAL;
+
+	if (copy_from_user(buf, userbuf, count))
+		return -EFAULT;
+
+	buf[sizeof(_buf) - 1] = '\0';
+	if (sscanf(buf, "queue limit %u %u %u", &ac, &q_limit_l, &q_limit_h)
+	    != 3)
+		return -EINVAL;
+
+	if (ac >= IEEE80211_NUM_ACS)
+		return -EINVAL;
+
+	sta->airtime[ac].aql_limit_low = q_limit_l;
+	sta->airtime[ac].aql_limit_high = q_limit_h;
 
 	for (ac = 0; ac < IEEE80211_NUM_ACS; ac++) {
 		spin_lock_bh(&local->active_txq_lock[ac]);