netdev - Re: [PATCH net-next v9 3/3] net: add sysfs attribute to control napi threaded mode

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Message-ID: <20210202162851.1ba89f80@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 2 Feb 2021 16:28:51 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Wei Wang <weiwan@...gle.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Felix Fietkau <nbd@....name>
Subject: Re: [PATCH net-next v9 3/3] net: add sysfs attribute to control
 napi threaded mode

On Fri, 29 Jan 2021 10:18:12 -0800 Wei Wang wrote:
> This patch adds a new sysfs attribute to the network device class.
> Said attribute provides a per-device control to enable/disable the
> threaded mode for all the napi instances of the given network device,
> without the need for a device up/down.
> User sets it to 1 or 0 to enable or disable threaded mode.
> 
> Co-developed-by: Paolo Abeni <pabeni@...hat.com>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> Co-developed-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Co-developed-by: Felix Fietkau <nbd@....name>
> Signed-off-by: Felix Fietkau <nbd@....name>
> Signed-off-by: Wei Wang <weiwan@...gle.com>

> +static int napi_set_threaded(struct napi_struct *n, bool threaded)
> +{
> +	int err = 0;
> +
> +	if (threaded == !!test_bit(NAPI_STATE_THREADED, &n->state))
> +		return 0;
> +
> +	if (!threaded) {
> +		clear_bit(NAPI_STATE_THREADED, &n->state);

Can we put a note in the commit message saying that stopping the
threads is slightly tricky but we'll do it if someone complains?

Or is there a stronger reason than having to wait for thread to finish
up with the NAPI not to stop them?

> +		return 0;
> +	}
> +
> +	if (!n->thread) {
> +		err = napi_kthread_create(n);
> +		if (err)
> +			return err;
> +	}
> +
> +	/* Make sure kthread is created before THREADED bit
> +	 * is set.
> +	 */
> +	smp_mb__before_atomic();
> +	set_bit(NAPI_STATE_THREADED, &n->state);
> +
> +	return 0;
> +}
> +
> +static void dev_disable_threaded_all(struct net_device *dev)
> +{
> +	struct napi_struct *napi;
> +
> +	list_for_each_entry(napi, &dev->napi_list, dev_list)
> +		napi_set_threaded(napi, false);
> +	dev->threaded = 0;
> +}
> +
> +int dev_set_threaded(struct net_device *dev, bool threaded)
> +{
> +	struct napi_struct *napi;
> +	int ret;
> +
> +	dev->threaded = threaded;
> +	list_for_each_entry(napi, &dev->napi_list, dev_list) {
> +		ret = napi_set_threaded(napi, threaded);
> +		if (ret) {
> +			/* Error occurred on one of the napi,
> +			 * reset threaded mode on all napi.
> +			 */
> +			dev_disable_threaded_all(dev);
> +			break;
> +		}
> +	}
> +
> +	return ret;
> +}
> +
>  void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
>  		    int (*poll)(struct napi_struct *, int), int weight)
>  {
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index daf502c13d6d..884f049ee395 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -538,6 +538,55 @@ static ssize_t phys_switch_id_show(struct device *dev,
>  }
>  static DEVICE_ATTR_RO(phys_switch_id);
>  
> +static ssize_t threaded_show(struct device *dev,
> +			     struct device_attribute *attr, char *buf)
> +{
> +	struct net_device *netdev = to_net_dev(dev);
> +	int ret;
> +
> +	if (!rtnl_trylock())
> +		return restart_syscall();
> +
> +	if (!dev_isalive(netdev)) {
> +		ret = -EINVAL;
> +		goto unlock;
> +	}
> +
> +	if (list_empty(&netdev->napi_list)) {
> +		ret = -EOPNOTSUPP;
> +		goto unlock;
> +	}

Maybe others disagree but I'd take this check out. What's wrong with
letting users see that threaded napi is disabled for devices without
NAPI?

This will also help a little devices which remove NAPIs when they are
down.

I've been caught off guard in the past by the fact that kernel returns
-ENOENT for XPS map when device has a single queue.

> +	ret = sprintf(buf, fmt_dec, netdev->threaded);
> +
> +unlock:
> +	rtnl_unlock();
> +	return ret;
> +}
> +
> +static int modify_napi_threaded(struct net_device *dev, unsigned long val)
> +{
> +	int ret;
> +
> +	if (list_empty(&dev->napi_list))
> +		return -EOPNOTSUPP;
> +
> +	if (val != 0 && val != 1)
> +		return -EOPNOTSUPP;
> +
> +	ret = dev_set_threaded(dev, val);
> +
> +	return ret;

return dev_set_threaded(dev, val);

> +}