Subject: Re: [PATCH net-next] net: sysfs: Do not create sysfs for non BQL device

From: Jakub Kicinski <[email-masked]>
Date: Thu, 15 Feb 2024 07:24:59 -0800

On Thu, 15 Feb 2024 03:27:27 -0800 Breno Leitao wrote:
> Creation of sysfs entries is expensive, mainly for workloads that
> constantly creates netdev and netns often.
> 
> Do not create BQL sysfs entries for devices that don't need,
> basically those that do not have a real queue, i.e, devices that has
> NETIF_F_LLTX and IFF_NO_QUEUE, such as `lo` interface.
> 
> This will remove the /sys/class/net/eth0/queues/tx-X/byte_queue_limits/
> directory for these devices.
> 
> In the example below, eth0 has the `byte_queue_limits` directory but not
> `lo`.
> 
> 	# ls /sys/class/net/lo/queues/tx-0/
> 	traffic_class  tx_maxrate  tx_timeout  xps_cpus  xps_rxqs
> 
> 	# ls /sys/class/net/eth0/queues/tx-0/byte_queue_limits/
> 	hold_time  inflight  limit  limit_max  limit_min

I'm tempted to also get rid of the #ifdefs while at it.

> +static bool netdev_uses_bql(struct net_device *dev)
> +{
> +	if (dev->features & NETIF_F_LLTX ||
> +	    dev->priv_flags & IFF_NO_QUEUE)
> +		return false;
> +
> +	return true;

make this
	return IS_ENABLED(CONFIG_BQL);

And throw in something like:

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index a09d507c5b03..119075dff0ee 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1454,6 +1454,9 @@ static const struct attribute_group dql_group = {
        .name  = "byte_queue_limits",
        .attrs  = dql_attrs,
 };
+#else
+/* Fake declaration, all the code using it should be dead */
+extern const struct attribute_group dql_group;
 #endif /* CONFIG_BQL */
 
 #ifdef CONFIG_XPS

You should then be able to remove the #ifdef CONFIG_BQL around the
uses of netdev_uses_bql(), compiler will realize it always returns
false and eliminate the code making use of &dql_group.