mac80211: improve latency and throughput while software scanning

Patch vastly improve latency while scanning. Slight throughput
improvements were observed as well. Is intended for improve performance
of voice and video applications, when scan is periodically requested by
user space (i.e. default NetworkManager behaviour).

Patch remove latency requirement based on PM_QOS_NETWORK_LATENCY,
this value is 2000 seconds by default (i.e. approximately 0.5 hour !?!).

Also remove listen interval requirement, which based on beaconing and
depending on BSS parameters. It can make we stay off-channel for a
second or more.

Instead try to offer the best latency that we could, i.e. be off-channel
no longer than PASSIVE channel scan time: 125 ms. That mean we will
scan two ACTIVE channels and go back to on-channel, and one PASSIVE
channel, and go back to on-channel.

Patch also decrease PASSIVE channel scan time to about 110 ms.

As drawback patch increase overall scan time. On my tests, when scanning
both 2GHz and 5GHz bands, scanning time increase from 5 seconds up to 10
seconds. Since that increase happen only when we are associated, I think
it can be acceptable. If eventually better scan time is needed for
situations when we lose signal and quickly need to decide to which AP
roam, additional scan flag or parameter can be introduced.

I tested patch by doing:

while true; do iw dev wlan0 scan; sleep 3; done > /dev/null

and

ping -i0.2 -c 1000 HOST

on remote and local machine, results are as below:

* Ping from local periodically scanning machine to AP:
Unpatched: rtt min/avg/max/mdev = 0.928/24.946/182.135/36.873 ms
Patched:   rtt min/avg/max/mdev = 0.928/19.678/150.845/33.130 ms

* Ping from remote machine to periodically scanning machine:
Unpatched: rtt min/avg/max/mdev = 1.637/120.683/709.139/164.337 ms
Patched:   rtt min/avg/max/mdev = 1.807/26.893/201.435/40.284 ms

Throughput measured by scp show following results.

* Upload to periodically scanning machine:
Unpatched: 3.9MB/s   03:15
Patched:   4.3MB/s   02:58

* Download from periodically scanning machine:
Unpatched: 5.5MB/s   02:17
Patched:   6.2MB/s   02:02
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/scan.c

@@ -27,7 +27,7 @@
 
 #define IEEE80211_PROBE_DELAY (HZ / 33)
 #define IEEE80211_CHANNEL_TIME (HZ / 33)
-#define IEEE80211_PASSIVE_CHANNEL_TIME (HZ / 8)
+#define IEEE80211_PASSIVE_CHANNEL_TIME (HZ / 9)
 
 void ieee80211_rx_bss_put(struct ieee80211_local *local,
 			  struct ieee80211_bss *bss)
@@ -535,8 +535,6 @@ static void ieee80211_scan_state_decision(struct ieee80211_local *local,
 	bool associated = false;
 	bool tx_empty = true;
 	bool bad_latency;
-	bool listen_int_exceeded;
-	unsigned long min_beacon_int = 0;
 	struct ieee80211_sub_if_data *sdata;
 	struct ieee80211_channel *next_chan;
 	enum mac80211_scan_state next_scan_state;
@@ -555,11 +553,6 @@ static void ieee80211_scan_state_decision(struct ieee80211_local *local,
 			if (sdata->u.mgd.associated) {
 				associated = true;
 
-				if (sdata->vif.bss_conf.beacon_int <
-				    min_beacon_int || min_beacon_int == 0)
-					min_beacon_int =
-						sdata->vif.bss_conf.beacon_int;
-
 				if (!qdisc_all_tx_empty(sdata->dev)) {
 					tx_empty = false;
 					break;
@@ -576,34 +569,19 @@ static void ieee80211_scan_state_decision(struct ieee80211_local *local,
 	 * see if we can scan another channel without interfering
 	 * with the current traffic situation.
 	 *
-	 * Since we don't know if the AP has pending frames for us
-	 * we can only check for our tx queues and use the current
-	 * pm_qos requirements for rx. Hence, if no tx traffic occurs
-	 * at all we will scan as many channels in a row as the pm_qos
-	 * latency allows us to. Additionally we also check for the
-	 * currently negotiated listen interval to prevent losing
-	 * frames unnecessarily.
-	 *
-	 * Otherwise switch back to the operating channel.
+	 * Keep good latency, do not stay off-channel more than 125 ms.
 	 */
 
 	bad_latency = time_after(jiffies +
 				 ieee80211_scan_get_channel_time(next_chan),
-			local->leave_oper_channel_time +
-			usecs_to_jiffies(pm_qos_request(PM_QOS_NETWORK_LATENCY)));
-
-	listen_int_exceeded = time_after(jiffies +
-			ieee80211_scan_get_channel_time(next_chan),
-			local->leave_oper_channel_time +
-			usecs_to_jiffies(min_beacon_int * 1024) *
-			local->hw.conf.listen_interval);
+				 local->leave_oper_channel_time + HZ / 8);
 
 	if (associated && !tx_empty) {
 		if (local->scan_req->flags & NL80211_SCAN_FLAG_LOW_PRIORITY)
 			next_scan_state = SCAN_ABORT;
 		else
 			next_scan_state = SCAN_SUSPEND;
-	} else if (associated && (bad_latency || listen_int_exceeded)) {
+	} else if (associated && bad_latency) {
 		next_scan_state = SCAN_SUSPEND;
 	} else {
 		next_scan_state = SCAN_SET_CHANNEL;