mac80211: optimize aggregation session timeout handling

Calling mod_timer from the rx/tx hotpath is somewhat expensive, and the
timeout doesn't need to be so precise.

Switch to a different strategy: Schedule the timer initially, store jiffies
of all last rx/tx activity which would previously modify the timer, and
let the timer re-arm itself after checking the last rx/tx timestamp.
Make the session timers deferrable to avoid causing extra wakeups on systems
running on battery.
This visibly reduces CPU load under high network load on small embedded
systems.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/sta_info.h

@@ -101,6 +101,7 @@ enum ieee80211_sta_info_flags {
  * @dialog_token: dialog token for aggregation session
  * @timeout: session timeout value to be filled in ADDBA requests
  * @state: session state (see above)
+ * @last_tx: jiffies of last tx activity
  * @stop_initiator: initiator of a session stop
  * @tx_stop: TX DelBA frame when stopping
  * @buf_size: reorder buffer size at receiver
@@ -122,6 +123,7 @@ struct tid_ampdu_tx {
 	struct timer_list addba_resp_timer;
 	struct sk_buff_head pending;
 	unsigned long state;
+	unsigned long last_tx;
 	u16 timeout;
 	u8 dialog_token;
 	u8 stop_initiator;
@@ -139,6 +141,7 @@ struct tid_ampdu_tx {
  * @reorder_time: jiffies when skb was added
  * @session_timer: check if peer keeps Tx-ing on the TID (by timeout value)
  * @reorder_timer: releases expired frames from the reorder buffer.
+ * @last_rx: jiffies of last rx activity
  * @head_seq_num: head sequence number in reordering buffer.
  * @stored_mpdu_num: number of MPDUs in reordering buffer
  * @ssn: Starting Sequence Number expected to be aggregated.
@@ -163,6 +166,7 @@ struct tid_ampdu_rx {
 	unsigned long *reorder_time;
 	struct timer_list session_timer;
 	struct timer_list reorder_timer;
+	unsigned long last_rx;
 	u16 head_seq_num;
 	u16 stored_mpdu_num;
 	u16 ssn;