sfc: only schedule asynchronous filter work if needed

Prevent excessive CPU time spent running a workitem with nothing to do.

We avoid any races by keeping the same check in efx_filter_rfs_expire().
Suggested-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/efx.h

@@ -159,7 +159,7 @@ static inline void efx_filter_rfs_expire(struct work_struct *data)
 	channel = container_of(dwork, struct efx_channel, filter_work);
 	time = jiffies - channel->rfs_last_expiry;
 	quota = channel->rfs_filter_count * time / (30 * HZ);
-	if (quota > 20 && __efx_filter_rfs_expire(channel, min(channel->rfs_filter_count, quota)))
+	if (quota >= 20 && __efx_filter_rfs_expire(channel, min(channel->rfs_filter_count, quota)))
 		channel->rfs_last_expiry += time;
 	/* Ensure we do more work eventually even if NAPI poll is not happening */
 	schedule_delayed_work(dwork, 30 * HZ);

drivers/net/ethernet/sfc/efx_channels.c

@@ -1166,6 +1166,9 @@ static int efx_poll(struct napi_struct *napi, int budget)
 	struct efx_channel *channel =
 		container_of(napi, struct efx_channel, napi_str);
 	struct efx_nic *efx = channel->efx;
+#ifdef CONFIG_RFS_ACCEL
+	unsigned int time;
+#endif
 	int spent;
 
 	netif_vdbg(efx, intr, efx->net_dev,
@@ -1185,7 +1188,10 @@ static int efx_poll(struct napi_struct *napi, int budget)
 
 #ifdef CONFIG_RFS_ACCEL
 		/* Perhaps expire some ARFS filters */
-		mod_delayed_work(system_wq, &channel->filter_work, 0);
+		time = jiffies - channel->rfs_last_expiry;
+		/* Would our quota be >= 20? */
+		if (channel->rfs_filter_count * time >= 600 * HZ)
+			mod_delayed_work(system_wq, &channel->filter_work, 0);
 #endif
 
 		/* There is no race here; although napi_disable() will