net: set the minimum for net_hotdata.netdev_budget_usecs

Commit 7acf8a1 ("Replace 2 jiffies with sysctl netdev_budget_usecs
to enable softirq tuning") added a possibility to set
net_hotdata.netdev_budget_usecs, but added no lower bound checking.

Commit a483798 ("net: revert default NAPI poll timeout to 2 jiffies")
made the *initial* value HZ-dependent, so the initial value is at least
2 jiffies even for lower HZ values (2 ms for 1000 Hz, 8ms for 250 Hz, 20
ms for 100 Hz).

But a user still can set improper values by a sysctl. Set .extra1
(the lower bound) for net_hotdata.netdev_budget_usecs to the same value
as in the latter commit. That is to 2 jiffies.

Fixes: a483798 ("net: revert default NAPI poll timeout to 2 jiffies")
Fixes: 7acf8a1 ("Replace 2 jiffies with sysctl netdev_budget_usecs to enable softirq tuning")
Signed-off-by: Jiri Slaby (SUSE) <[email protected]>
Cc: Dmitry Yakunin <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Jiri Slaby (SUSE)

net/core/sysctl_net_core.c

@@ -34,6 +34,7 @@ static int min_sndbuf = SOCK_MIN_SNDBUF;
 static int min_rcvbuf = SOCK_MIN_RCVBUF;
 static int max_skb_frags = MAX_SKB_FRAGS;
 static int min_mem_pcpu_rsv = SK_MEMORY_PCPU_RESERVE;
+static int netdev_budget_usecs_min = 2 * USEC_PER_SEC / HZ;
 
 static int net_msg_warn;	/* Unused, but still a sysctl */
 
@@ -587,7 +588,7 @@ static struct ctl_table net_core_table[] = {
 		.maxlen		= sizeof(unsigned int),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
-		.extra1		= SYSCTL_ZERO,
+		.extra1		= &netdev_budget_usecs_min,
 	},
 	{
 		.procname	= "fb_tunnels_only_for_init_net",