netdev - Re: [PATCH net-next 5/5] net: improve napi threaded config

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Date:   Thu, 1 Oct 2020 12:01:03 +0200
From:   Felix Fietkau <nbd@....name>
To:     Wei Wang <weiwan@...gle.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next 5/5] net: improve napi threaded config


On 2020-09-30 21:21, Wei Wang wrote:
> This commit mainly addresses the threaded config to make the switch
> between softirq based and kthread based NAPI processing not require
> a device down/up.
> It also moves the kthread_create() call to the sysfs handler when user
> tries to enable "threaded" on napi, and properly handles the
> kthread_create() failure. This is because certain drivers do not have
> the napi created and linked to the dev when dev_open() is called. So
> the previous implementation does not work properly there.
> 
> Signed-off-by: Wei Wang <weiwan@...gle.com>
> ---
> Changes since RFC:
> changed the thread name to napi/<dev>-<napi-id>
> 
>  net/core/dev.c       | 49 +++++++++++++++++++++++++-------------------
>  net/core/net-sysfs.c |  9 +++-----
>  2 files changed, 31 insertions(+), 27 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b4f33e442b5e..bf878d3a9d89 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1490,17 +1490,24 @@ EXPORT_SYMBOL(netdev_notify_peers);
>  
>  static int napi_threaded_poll(void *data);
>  
> -static void napi_thread_start(struct napi_struct *n)
> +static int napi_kthread_create(struct napi_struct *n)
>  {
> -	if (test_bit(NAPI_STATE_THREADED, &n->state) && !n->thread)
> -		n->thread = kthread_create(napi_threaded_poll, n, "%s-%d",
> -					   n->dev->name, n->napi_id);
> +	int err = 0;
> +
> +	n->thread = kthread_create(napi_threaded_poll, n, "napi/%s-%d",
> +				   n->dev->name, n->napi_id);
> +	if (IS_ERR(n->thread)) {
> +		err = PTR_ERR(n->thread);
> +		pr_err("kthread_create failed with err %d\n", err);
> +		n->thread = NULL;
> +	}
> +
> +	return err;
If I remember correctly, using kthread_create with no explicit first
wakeup means the task will sit there and contribute to system loadavg
until it is woken up the first time.
Shouldn't we use kthread_run here instead?

- Felix