netdev: fix repeated netlink messages in queue stats

The context is supposed to record the next queue to dump,
not last dumped. If the dump doesn't fit we will restart
from the already-dumped queue, duplicating the message.

Before this fix and with the selftest improvements later
in this series we see:

  # ./run_kselftest.sh -t drivers/net:stats.py
  timeout set to 45
  selftests: drivers/net: stats.py
  KTAP version 1
  1..5
  ok 1 stats.check_pause
  ok 2 stats.check_fec
  ok 3 stats.pkt_byte_sum
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 125, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), len(set(queues[qtype])),
  # Check failed 45 != 44 repeated queue keys
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 127, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), max(queues[qtype]) + 1,
  # Check failed 45 != 44 missing queue keys
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 125, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), len(set(queues[qtype])),
  # Check failed 45 != 44 repeated queue keys
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 127, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), max(queues[qtype]) + 1,
  # Check failed 45 != 44 missing queue keys
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 125, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), len(set(queues[qtype])),
  # Check failed 103 != 100 repeated queue keys
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 127, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), max(queues[qtype]) + 1,
  # Check failed 103 != 100 missing queue keys
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 125, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), len(set(queues[qtype])),
  # Check failed 102 != 100 repeated queue keys
  # Check| At /root/ksft-net-drv/drivers/net/./stats.py, line 127, in qstat_by_ifindex:
  # Check|     ksft_eq(len(queues[qtype]), max(queues[qtype]) + 1,
  # Check failed 102 != 100 missing queue keys
  not ok 4 stats.qstat_by_ifindex
  ok 5 stats.check_down
  # Totals: pass:4 fail:1 xfail:0 xpass:0 skip:0 error:0

With the fix:

  # ./ksft-net-drv/run_kselftest.sh -t drivers/net:stats.py
  timeout set to 45
  selftests: drivers/net: stats.py
  KTAP version 1
  1..5
  ok 1 stats.check_pause
  ok 2 stats.check_fec
  ok 3 stats.pkt_byte_sum
  ok 4 stats.qstat_by_ifindex
  ok 5 stats.check_down
  # Totals: pass:5 fail:0 xfail:0 xpass:0 skip:0 error:0

Fixes: ab63a23 ("netdev: add per-queue statistics")
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20241213152244.3080955-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

net/core/netdev-genl.c

@@ -668,15 +668,15 @@ netdev_nl_stats_by_queue(struct net_device *netdev, struct sk_buff *rsp,
 					    i, info);
 		if (err)
 			return err;
-		ctx->rxq_idx = i++;
+		ctx->rxq_idx = ++i;
 	}
 	i = ctx->txq_idx;
 	while (ops->get_queue_stats_tx && i < netdev->real_num_tx_queues) {
 		err = netdev_nl_stats_queue(netdev, rsp, NETDEV_QUEUE_TYPE_TX,
 					    i, info);
 		if (err)
 			return err;
-		ctx->txq_idx = i++;
+		ctx->txq_idx = ++i;
 	}
 
 	ctx->rxq_idx = 0;