[RFC -next 2/2] net: sched: red: don't reset the backlog on every stat dump

[Jakub Kicinski <jakub.kicinski@netronome.com>]

Commit 0dfb33a0d7e2 ("sch_red: report backlog information") copied
child's backlog into RED's backlog.  Back then RED did not maintain
its own backlog counts.  This has changed after commit 2ccccf5fb43f
("net_sched: update hierarchical backlog too") and commit d7f4f332f082
("sch_red: update backlog as well").  Copying is no longer necessary.

Tested:

$ tc -s qdisc show dev veth0
qdisc red 1: root refcnt 2 limit 400000b min 30000b max 30000b ecn
 Sent 20942 bytes 221 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 1260b 14p requeues 14
  marked 0 early 0 pdrop 0 other 0
qdisc tbf 2: parent 1: rate 1Kbit burst 15000b lat 3585.0s
 Sent 20942 bytes 221 pkt (dropped 0, overlimits 138 requeues 0)
 backlog 1260b 14p requeues 14

Recently RED offload was added.  We need to make sure drivers don't
depend on resetting the stats.  This means backlog should be treated
like any other statistic:

  total_stat = new_hw_stat - prev_hw_stat;

Unlike for other statistics new_hw_stat < prev_hw_stat can be true.
Adjust mlxsw.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
---
 drivers/net/ethernet/mellanox/mlxsw/spectrum.h       | 1 +
 drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c | 9 +++++++--
 net/sched/sch_red.c                                  | 1 -
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlxsw/spectrum.h b/drivers/net/ethernet/mellanox/mlxsw/spectrum.h
index ff8d32bc852c..6755050e4ee0 100644
--- a/drivers/net/ethernet/mellanox/mlxsw/spectrum.h
+++ b/drivers/net/ethernet/mellanox/mlxsw/spectrum.h
@@ -237,6 +237,7 @@ struct mlxsw_sp_qdisc {
 	u64 tx_packets;
 	u64 drops;
 	u64 overlimits;
+	u64 backlog;
 };
 
 /* No need an internal lock; At worse - miss a single periodic iteration */
diff --git a/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c b/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
index c33beac5def0..d5091740bd40 100644
--- a/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
+++ b/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
@@ -212,6 +212,7 @@ mlxsw_sp_qdisc_get_red_stats(struct mlxsw_sp_port *mlxsw_sp_port, u32 handle,
 	u64 tx_bytes, tx_packets, overlimits, drops;
 	struct mlxsw_sp_port_xstats *xstats;
 	struct rtnl_link_stats64 *stats;
+	s64 backlog;
 
 	if (mlxsw_sp_qdisc->handle != handle ||
 	    mlxsw_sp_qdisc->type != MLXSW_SP_QDISC_RED)
@@ -226,17 +227,21 @@ mlxsw_sp_qdisc_get_red_stats(struct mlxsw_sp_port *mlxsw_sp_port, u32 handle,
 		     mlxsw_sp_qdisc->overlimits;
 	drops = xstats->wred_drop[tclass_num] + xstats->tail_drop[tclass_num] -
 		mlxsw_sp_qdisc->drops;
+	backlog = mlxsw_sp_cells_bytes(mlxsw_sp_port->mlxsw_sp,
+				       xstats->backlog[tclass_num]) -
+		mlxsw_sp_qdisc->backlog;
 
 	_bstats_update(res->bstats, tx_bytes, tx_packets);
 	res->qstats->overlimits += overlimits;
 	res->qstats->drops += drops;
-	res->qstats->backlog += mlxsw_sp_cells_bytes(mlxsw_sp_port->mlxsw_sp,
-						xstats->backlog[tclass_num]);
+	res->qstats->backlog +=	backlog;
 
 	mlxsw_sp_qdisc->drops +=  drops;
 	mlxsw_sp_qdisc->overlimits += overlimits;
 	mlxsw_sp_qdisc->tx_bytes += tx_bytes;
 	mlxsw_sp_qdisc->tx_packets += tx_packets;
+	mlxsw_sp_qdisc->backlog += backlog;
+
 	return 0;
 }
 
diff --git a/net/sched/sch_red.c b/net/sched/sch_red.c
index a392eaa4a0b4..caebb7e37551 100644
--- a/net/sched/sch_red.c
+++ b/net/sched/sch_red.c
@@ -322,7 +322,6 @@ static int red_dump(struct Qdisc *sch, struct sk_buff *skb)
 	};
 	int err;
 
-	sch->qstats.backlog = q->qdisc->qstats.backlog;
 	err = red_dump_offload_stats(sch, &opt);
 	if (err)
 		goto nla_put_failure;
-- 
2.15.1