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>

1 files changed, 16 insertions, 2 deletions

diff --git a/net/mac80211/agg-tx.c b/net/mac80211/agg-tx.c
index 76be61744198..9628a1892441 100644
--- a/net/mac80211/agg-tx.c
+++ b/net/mac80211/agg-tx.c
@@ -417,6 +417,18 @@ static void sta_tx_agg_session_timer_expired(unsigned long data)
         u8 *timer_to_id = ptid - *ptid;
         struct sta_info *sta = container_of(timer_to_id, struct sta_info,
                                          timer_to_tid[0]);
+        struct tid_ampdu_tx *tid_tx;
+        unsigned long timeout;
+
+        tid_tx = rcu_dereference_protected_tid_tx(sta, *ptid);
+        if (!tid_tx)
+                return;
+
+        timeout = tid_tx->last_tx + TU_TO_JIFFIES(tid_tx->timeout);
+        if (time_is_after_jiffies(timeout)) {
+                mod_timer(&tid_tx->session_timer, timeout);
+                return;
+        }
 
 #ifdef CONFIG_MAC80211_HT_DEBUG
         printk(KERN_DEBUG "tx session timer expired on tid %d\n", (u16)*ptid);
@@ -542,7 +554,7 @@ int ieee80211_start_tx_ba_session(struct ieee80211_sta *pubsta, u16 tid,
         /* tx timer */
         tid_tx->session_timer.function = sta_tx_agg_session_timer_expired;
         tid_tx->session_timer.data = (unsigned long)&sta->timer_to_tid[tid];
-        init_timer(&tid_tx->session_timer);
+        init_timer_deferrable(&tid_tx->session_timer);
 
         /* assign a dialog token */
         sta->ampdu_mlme.dialog_token_allocator++;
@@ -884,9 +896,11 @@ void ieee80211_process_addba_resp(struct ieee80211_local *local,
 
                 sta->ampdu_mlme.addba_req_num[tid] = 0;
 
-                if (tid_tx->timeout)
+                if (tid_tx->timeout) {
                         mod_timer(&tid_tx->session_timer,
                                   TU_TO_EXP_TIME(tid_tx->timeout));
+                        tid_tx->last_tx = jiffies;
+                }
 
         } else {
                 ___ieee80211_stop_tx_ba_session(sta, tid, WLAN_BACK_INITIATOR,