[PATCH net-next] selftests: drv-net: Limit the max number of queues in procfs_downup_hammer

[Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>]

For NICs with a large (1024+) number of queues, this test can cause
excessive memory fragmentation. This results in OOM errors, and in the
worst case driver/kernel crashes. We don't need to test with the max number
of queues, just enough to create a high likelihood of races between
reconfiguration and stats getting read.

Signed-off-by: Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 tools/testing/selftests/drivers/net/stats.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/drivers/net/stats.py b/tools/testing/selftests/drivers/net/stats.py
index 04d0a2a13e73..b08e4d48b15c 100755
--- a/tools/testing/selftests/drivers/net/stats.py
+++ b/tools/testing/selftests/drivers/net/stats.py
@@ -263,14 +263,15 @@ def procfs_downup_hammer(cfg) -> None:
     Reading stats via procfs only holds the RCU lock, drivers often try
     to sleep when reading the stats, or don't protect against races.
     """
-    # Max out the queues, we'll flip between max and 1
+    # Set a large number of queues,
+    # we'll flip between min(max_queues, 64) and 1
     channels = ethnl.channels_get({'header': {'dev-index': cfg.ifindex}})
     if channels['combined-count'] == 0:
         rx_type = 'rx'
     else:
         rx_type = 'combined'
     cur_queue_cnt = channels[f'{rx_type}-count']
-    max_queue_cnt = channels[f'{rx_type}-max']
+    max_queue_cnt = min(channels[f'{rx_type}-max'], 64)
 
     cmd(f"ethtool -L {cfg.ifname} {rx_type} {max_queue_cnt}")
     defer(cmd, f"ethtool -L {cfg.ifname} {rx_type} {cur_queue_cnt}")
-- 
2.47.3