netdev - Re: [PATCH v2] net: add support for threaded NAPI polling

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Date:   Thu, 6 Aug 2020 10:39:30 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Felix Fietkau <nbd@....name>, netdev@...r.kernel.org
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v2] net: add support for threaded NAPI polling



On 8/6/20 2:55 AM, Felix Fietkau wrote:
> For some drivers (especially 802.11 drivers), doing a lot of work in the NAPI
> poll function does not perform well. Since NAPI poll is bound to the CPU it
> was scheduled from, we can easily end up with a few very busy CPUs spending
> most of their time in softirq/ksoftirqd and some idle ones.
> 
> Introduce threaded NAPI for such drivers based on a workqueue. The API is the
> same except for using netif_threaded_napi_add instead of netif_napi_add.
> 
> In my tests with mt76 on MT7621 using threaded NAPI + a thread for tx scheduling
> improves LAN->WLAN bridging throughput by 10-50%. Throughput without threaded
> NAPI is wildly inconsistent, depending on the CPU that runs the tx scheduling
> thread.
> 
> With threaded NAPI, throughput seems stable and consistent (and higher than
> the best results I got without it).
> 
> Based on a patch by Hillf Danton
> 
> Cc: Hillf Danton <hdanton@...a.com>
> Signed-off-by: Felix Fietkau <nbd@....name>

...

> index e353b822bb15..99233e86f4c5 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -471,6 +471,47 @@ static ssize_t proto_down_store(struct device *dev,
>  }
>  NETDEVICE_SHOW_RW(proto_down, fmt_dec);
>  


This belongs to a separate patch, with correct attribution.

> +static int change_napi_threaded(struct net_device *dev, unsigned long val)
> +{
> +	struct napi_struct *napi;
> +
> +	if (list_empty(&dev->napi_list))
> +		return -EOPNOTSUPP;
> +	list_for_each_entry(napi, &dev->napi_list, dev_list) {
> +		if (val)
> +			set_bit(NAPI_STATE_THREADED, &napi->state);
> +		else
> +			clear_bit(NAPI_STATE_THREADED, &napi->state);
> +	}
> +
> +	return 0;
> +}
> +
> +static ssize_t napi_threaded_store(struct device *dev,
> +				struct device_attribute *attr,
> +				const char *buf, size_t len)
> +{
> +	return netdev_store(dev, attr, buf, len, change_napi_threaded);
> +}
> +
> +static ssize_t napi_threaded_show(struct device *dev,
> +				  struct device_attribute *attr,
> +				  char *buf)
> +{
> +	struct net_device *netdev = to_net_dev(dev);
> +	struct napi_struct *napi;
> +	bool enabled = false;
> +


You probably want to use RTNL protection, list could change under us otherwise.

The write side part is protected already in netdev_store()


> +	list_for_each_entry(napi, &netdev->napi_list, dev_list) {
> +		if (test_bit(NAPI_STATE_THREADED, &napi->state))
> +			enabled = true;
> +	}
> +
> +	return sprintf(buf, fmt_dec, enabled);
> +}
> +DEVICE_ATTR_RW(napi_threaded);
> +
>  static ssize_t phys_port_id_show(struct device *dev,
>  				 struct device_attribute *attr, char *buf)
>  {
> @@ -563,6 +604,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
>  	&dev_attr_tx_queue_len.attr,
>  	&dev_attr_gro_flush_timeout.attr,
>  	&dev_attr_napi_defer_hard_irqs.attr,
> +	&dev_attr_napi_threaded.attr,
>  	&dev_attr_phys_port_id.attr,
>  	&dev_attr_phys_port_name.attr,
>  	&dev_attr_phys_switch_id.attr,
>