Re: [RFC v3] net/core: add optional threading for backlog processing

[Felix Fietkau <nbd@nbd.name>]

On 19.02.23 14:10, Felix Fietkau wrote:
> When dealing with few flows or an imbalance on CPU utilization, static RPS
> CPU assignment can be too inflexible. Add support for enabling threaded NAPI
> for backlog processing in order to allow the scheduler to better balance
> processing. This helps better spread the load across idle CPUs.
> 
> Signed-off-by: Felix Fietkau <nbd@nbd.name>
> ---
> RFC v3:
>   - make patch more generic, applies to backlog processing in general
>   - fix process queue access on flush
> RFC v2:
>   - fix rebase error in rps locking
> 
>   include/linux/netdevice.h  |  2 +
>   net/core/dev.c             | 78 +++++++++++++++++++++++++++++++++++---
>   net/core/sysctl_net_core.c | 27 +++++++++++++
>   3 files changed, 102 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index d9cdbc047b49..b3cef91b1696 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -522,6 +522,7 @@ static inline bool napi_complete(struct napi_struct *n)
>   }
>   
>   int dev_set_threaded(struct net_device *dev, bool threaded);
> +int backlog_set_threaded(bool threaded);
>   
>   /**
>    *	napi_disable - prevent NAPI from scheduling
> @@ -3192,6 +3193,7 @@ struct softnet_data {
>   	unsigned int		cpu;
>   	unsigned int		input_queue_tail;
>   #endif
> +	unsigned int		process_queue_empty;
>   	unsigned int		received_rps;
>   	unsigned int		dropped;
>   	struct sk_buff_head	input_pkt_queue;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 357081b0113c..76874513b7b5 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4597,7 +4597,7 @@ static int napi_schedule_rps(struct softnet_data *sd)
>   	struct softnet_data *mysd = this_cpu_ptr(&softnet_data);
>   
>   #ifdef CONFIG_RPS
> -	if (sd != mysd) {
> +	if (sd != mysd && !test_bit(NAPI_STATE_THREADED, &sd->backlog.state)) {
>   		sd->rps_ipi_next = mysd->rps_ipi_list;
>   		mysd->rps_ipi_list = sd;
>   
> @@ -5778,6 +5778,8 @@ static DEFINE_PER_CPU(struct work_struct, flush_works);
>   /* Network device is going away, flush any packets still pending */
>   static void flush_backlog(struct work_struct *work)
>   {
> +	unsigned int process_queue_empty;
> +	bool threaded, flush_processq;
>   	struct sk_buff *skb, *tmp;
>   	struct softnet_data *sd;
>   
> @@ -5792,8 +5794,15 @@ static void flush_backlog(struct work_struct *work)
>   			input_queue_head_incr(sd);
>   		}
>   	}
> +
> +	threaded = test_bit(NAPI_STATE_THREADED, &sd->backlog.state);
> +	flush_processq = threaded &&
> +			 !skb_queue_empty_lockless(&sd->process_queue);
Sorry, the patch was missing these lines:
	if (flush_processq)
		process_queue_empty = sd->process_queue_empty;

>   	rps_unlock_irq_enable(sd);
>   
> +	if (threaded)
> +		goto out;
> +
>   	skb_queue_walk_safe(&sd->process_queue, skb, tmp) {
>   		if (skb->dev->reg_state == NETREG_UNREGISTERING) {
>   			__skb_unlink(skb, &sd->process_queue);