On Wed, 2013-01-23 at 12:32 +0100, Stanislaw Gruszka wrote:
> 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.
Total scan time isn't a problem as long as the latency of streaming
applications is improved, since that was the original problem anyway.
So I don't think it's a drawback.
Dan
> 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@xxxxxxxxxx>
> ---
> net/mac80211/scan.c | 32 +++++---------------------------
> 1 files changed, 5 insertions(+), 27 deletions(-)
>
> diff --git a/net/mac80211/scan.c b/net/mac80211/scan.c
> index 06cbe26..505699b 100644
> --- a/net/mac80211/scan.c
> +++ b/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)
>
> static void ieee80211_rx_bss_free(struct cfg80211_bss *cbss)
> {
> @@ -546,8 +546,6 @@ 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;
> @@ -566,11 +564,6 @@ 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;
> @@ -587,34 +580,19 @@ 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);
> + ieee80211_scan_get_channel_time(next_chan),
> + 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;
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