[PATCH] bgscan_simpl: Refinements to fast-scan backoff

[Paul Stewart]

These changes account for situations where the CQM threshold
might be approximately the same as the currently received signal,
and thus CQM events are triggered often due to measurement
error/small fluctuations.  This results in scanning occurring
too frequently.

Firstly, inhibit the immediate scan when the short-scan count
is at the maximum.  This keeps bursts of CQM toggling from
causing a torrent of back-to-back scans.  This does not inhibit
immediate scans if the CQM triggers a second time (if the signal
falls lower past the hysteresis).  This reduces the scan rate in
the worst case (fast-rate toggling high/low CQM events) to the
short scan interval.

Secondly, change the behavior of the short scan count so it acts like
a "leaky bucket".  As we perform short-scans, the bucket fills until
it reaches a maximal short-scan count, at which we back-off and
revert to a long scan interval.  The short scan count decreases by
one (emptying the bucket) every time we complete a long scan interval
without a low-RSSI CQM event.

This reduces the impact of medium-rate toggling of high/low CQM
events, reducing the number of short-interval scans that occur before
returning to a long-interval if the system was recently doing
short scans.
---
 wpa_supplicant/bgscan_simple.c |   23 +++++++++++++++++++----
 1 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/wpa_supplicant/bgscan_simple.c b/wpa_supplicant/bgscan_simple.c
index eedc961..47b1047 100644
--- a/wpa_supplicant/bgscan_simple.c
+++ b/wpa_supplicant/bgscan_simple.c
@@ -29,6 +29,7 @@ struct bgscan_simple_data {
 	int scan_interval;
 	int signal_threshold;
 	int short_scan_count; /* counter for scans using short scan interval */
+	int max_short_scans; /* maximum times we short-scan before back-off */
 	int short_interval; /* use if signal < threshold */
 	int long_interval; /* use if signal > threshold */
 	struct os_time last_bgscan;
@@ -66,12 +67,19 @@ static void bgscan_simple_timeout(void *eloop_ctx, void *timeout_ctx)
 			 * scanning at the short scan interval. After that,
 			 * revert to the long scan interval.
 			 */
-			if (data->short_scan_count >
-			    data->long_interval / data->short_interval + 1) {
+			if (data->short_scan_count > data->max_short_scans) {
 				data->scan_interval = data->long_interval;
 				wpa_printf(MSG_DEBUG, "bgscan simple: Backing "
 					   "off to long scan interval");
 			}
+		} else if (data->short_scan_count > 0) {
+			/*
+			 * If we lasted a long scan interval without any
+			 * CQM triggers, decrease the short-scan count,
+			 * which allows 1 more short-scan interval to
+			 * occur in the future when CQM triggers.
+			 */
+			data->short_scan_count--;
 		}
 		os_get_time(&data->last_bgscan);
 	}
@@ -138,6 +146,7 @@ static void * bgscan_simple_init(struct wpa_supplicant *wpa_s,
 	}
 
 	data->scan_interval = data->short_interval;
+	data->max_short_scans = data->long_interval / data->short_interval + 1;
 	if (data->signal_threshold) {
 		/* Poll for signal info to set initial scan interval */
 		struct wpa_signal_info siginfo;
@@ -222,9 +231,15 @@ static void bgscan_simple_notify_signal_change(void *priv, int above,
 		wpa_printf(MSG_DEBUG, "bgscan simple: Start using short "
 			   "bgscan interval");
 		data->scan_interval = data->short_interval;
-		data->short_scan_count = 0;
 		os_get_time(&now);
-		if (now.sec > data->last_bgscan.sec + 1)
+		if (now.sec > data->last_bgscan.sec + 1 &&
+		    data->short_scan_count <= data->max_short_scans)
+			/*
+			 * If we haven't just previously (<1 second ago)
+			 * preformed a scan, and we haven't depleted our
+			 * budget for short-scans, perform a scan
+			 * immediately.
+			 */
 			scan = 1;
 		else if (data->last_bgscan.sec + data->long_interval >
 			 now.sec + data->scan_interval) {
-- 
1.7.3.1