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bgscan_simple: Refinements to fast-scan backoff

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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.
This commit is contained in:
Paul Stewart 2012-02-05 18:43:06 +02:00 committed by Jouni Malinen
parent 0f4668ceac
commit 87343c2017
1 changed files with 19 additions and 4 deletions
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23
wpa_supplicant/bgscan_simple.c
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@ -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)
* performed 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) {