netdev - Re: [PATCH] rps: process the skb directly if rps cpu not changed

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Message-ID: <20230321195459.390dea45@kernel.org>
Date:   Tue, 21 Mar 2023 19:54:59 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     <yang.yang29@....com.cn>
Cc:     <edumazet@...gle.com>, <davem@...emloft.net>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <xu.xin16@....com.cn>, <jiang.xuexin@....com.cn>,
        <zhang.yunkai@....com.cn>
Subject: Re: [PATCH] rps: process the skb directly if rps cpu not changed

On Tue, 21 Mar 2023 20:12:29 +0800 (CST) yang.yang29@....com.cn wrote:
> The measured result shows the patch brings 50% reduction of NET_RX softirqs.
> The test was done on the QEMU environment with two-core CPU by iperf3.
> taskset 01 iperf3 -c 192.168.2.250 -t 3 -u -R;
> taskset 02 iperf3 -c 192.168.2.250 -t 3 -u -R;
> 
> Previous RPS:
> 		    	CPU0       CPU1

this header looks misalinged

> NET_RX:         45          0    (before iperf3 testing)
> NET_RX:        1095         241   (after iperf3 testing)
> 
> Patched RPS:
>                 CPU0       CPU1
> NET_RX:         28          4    (before iperf3 testing)
> NET_RX:         573         32   (after iperf3 testing)

This table is really confusing. What's the unit, how is it measured 
and why are you showing before/after rather than the delta?

> diff --git a/net/core/dev.c b/net/core/dev.c
> index c7853192563d..c33ddac3c012 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5666,8 +5666,9 @@ static int netif_receive_skb_internal(struct sk_buff *skb)
>  	if (static_branch_unlikely(&rps_needed)) {
>  		struct rps_dev_flow voidflow, *rflow = &voidflow;
>  		int cpu = get_rps_cpu(skb->dev, skb, &rflow);
> +		int current_cpu = smp_processor_id();
> 
> -		if (cpu >= 0) {
> +		if (cpu >= 0 && cpu != current_cpu) {
>  			ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
>  			rcu_read_unlock();
>  			return ret;
> @@ -5699,8 +5700,9 @@ void netif_receive_skb_list_internal(struct list_head *head)
>  		list_for_each_entry_safe(skb, next, head, list) {
>  			struct rps_dev_flow voidflow, *rflow = &voidflow;
>  			int cpu = get_rps_cpu(skb->dev, skb, &rflow);
> +			int current_cpu = smp_processor_id();

This does not have to be in the loop.

> 
> -			if (cpu >= 0) {
> +			if (cpu >= 0 && cpu != current_cpu) {

Please answer Yunsheng's question as well..