Subject: Re: RFC: ixgbe+build_skb+extra performance experiments

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 2 Oct 2014 09:36:36 +0200

On Wed,  1 Oct 2014 23:00:42 -0700 Alexei Starovoitov <[email-masked]> wrote:

> I'm trying to speed up single core packet per second.

Great, welcome to the club ;-)
 
> I took dual port ixgbe and added both ports to a linux bridge.
> Only one port is connected to another system running pktgen at 10G rate.
> I disabled gro to measure pure RX speed of ixgbe.

It is great that you are attacking the RX side, I planned to look at it
after finishing the qdisc bulking.  It is really lacking behind,
especially after we have now almost "fixed" the TX side (driver layer
can now do 14.8Mpps, if ignoring rest of stack, alloc etc.).

> Out of the box I see 6.5 Mpps and the following stack:
>   21.83%    ksoftirqd/0  [kernel.kallsyms]  [k] memcpy
>   17.58%    ksoftirqd/0  [ixgbe]            [k] ixgbe_clean_rx_irq
>   10.07%    ksoftirqd/0  [kernel.kallsyms]  [k] build_skb
>    6.40%    ksoftirqd/0  [kernel.kallsyms]  [k] __netdev_alloc_frag
>    5.18%    ksoftirqd/0  [kernel.kallsyms]  [k] put_compound_page
>    4.93%    ksoftirqd/0  [kernel.kallsyms]  [k] kmem_cache_alloc
>    4.55%    ksoftirqd/0  [kernel.kallsyms]  [k] __netif_receive_skb_core
> 
> Obviously driver spends huge amount of time copying data from
> hw buffers into skb.
> 
> Then I applied buggy but working in this case patch:
> http://patchwork.ozlabs.org/patch/236044/
> that is trying to use build_skb() API in ixgbe.

I also expected it will be a huge win to use build_skb() API.
Good to see this confirmed! :-)

I've been playing with a faster memory pool/allocator (implemented via a
ring_queue), and my experiments show I could save 52ns when using it
for the skb->data.  And you basically avoid this skb->data alloc with
build_skb().

> RX speed jumped to 7.6 Mpps with the following stack:
>   27.02%    ksoftirqd/0  [kernel.kallsyms]  [k] eth_type_trans
>   16.68%    ksoftirqd/0  [ixgbe]            [k] ixgbe_clean_rx_irq
>   11.45%    ksoftirqd/0  [kernel.kallsyms]  [k] build_skb
>    5.20%    ksoftirqd/0  [kernel.kallsyms]  [k] __netif_receive_skb_core
>    4.72%    ksoftirqd/0  [kernel.kallsyms]  [k] kmem_cache_alloc
>    3.96%    ksoftirqd/0  [kernel.kallsyms]  [k] kmem_cache_free

My faster memory pool/allocator could save 8ns for the kmem_cache/slub
calls, which is also high in your perf top.  8ns out of 40ns which is
the micro benchmarked cost of the kmem_cache_{alloc,free} calls.

> packets no longer copied and performance is higher.
> It's doing the following:
> - build_skb out of hw buffer and prefetch packet data
> - eth_type_trans
> - napi_gro_receive
> 
> but build_skb() is too fast and cpu doesn't have enough time
> to prefetch packet data before eth_type_trans() is called,
> so I added mini skb bursting of 2 skbs (patch below) that does:
> - build_skb1 out of hw buffer and prefetch packet data
> - build_skb2 out of hw buffer and prefetch packet data
> - eth_type_trans(skb1)
> - napi_gro_receive(skb1)
> - eth_type_trans(skb2)
> - napi_gro_receive(skb2)
> and performance jumped to 9.0 Mpps with stack:
>   20.54%    ksoftirqd/0  [ixgbe]            [k] ixgbe_clean_rx_irq
>   13.15%    ksoftirqd/0  [kernel.kallsyms]  [k] build_skb
>    8.35%    ksoftirqd/0  [kernel.kallsyms]  [k] __netif_receive_skb_core
>    7.16%    ksoftirqd/0  [kernel.kallsyms]  [k] eth_type_trans
>    4.73%    ksoftirqd/0  [kernel.kallsyms]  [k] kmem_cache_free
>    4.50%    ksoftirqd/0  [kernel.kallsyms]  [k] kmem_cache_alloc
> 
> with further instruction tunning inside ixgbe_clean_rx_irq()
> I could push it to 9.4 Mpps.
> 
> From 6.5 Mpps to 9.4 Mpps via build_skb() and tunning.

Cool, quite impressive performance boost! - good work! :-)

> Is there a way to fix the issue Ben pointed a year ago?
> Brute force fix could to be: avoid half-page buffers.
> We'll be wasting 16Mbyte of memory. Sure, but in some cases
> extra peformance might be worth it.
> Other options?
>
> I think we need to try harder to switch to build_skb()
> It will open up a lot of possibilities for further performance
> improvements.
> Thoughts?

Yes, we should really work on converting more drivers to use
build_skb().

> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |   34 +++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index 21d1a65..1d1e37f 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -1590,8 +1590,6 @@ static void ixgbe_process_skb_fields(struct ixgbe_ring *rx_ring,
>  	}
> 
>  	skb_record_rx_queue(skb, rx_ring->queue_index);
> -
> -	skb->protocol = eth_type_trans(skb, dev);
>  }
> 
>  static void ixgbe_rx_skb(struct ixgbe_q_vector *q_vector,
> @@ -2063,6 +2061,24 @@ dma_sync:
>  	return skb;
>  }
> 
> +#define BURST_SIZE 2
> +static void ixgbe_rx_skb_burst(struct sk_buff *skbs[BURST_SIZE],
> +			       unsigned int skb_burst,
> +			       struct ixgbe_q_vector *q_vector,
> +			       struct net_device *dev)
> +{
> +	int i;
> +
> +	for (i = 0; i < skb_burst; i++) {
> +		struct sk_buff *skb = skbs[i];
> +
> +		skb->protocol = eth_type_trans(skb, dev);
> +
> +		skb_mark_napi_id(skb, &q_vector->napi);
> +		ixgbe_rx_skb(q_vector, skb);
> +	}
> +}
> +
>  /**
>   * ixgbe_clean_rx_irq - Clean completed descriptors from Rx ring - bounce buf
>   * @q_vector: structure containing interrupt and ring information
> @@ -2087,6 +2103,8 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
>  	unsigned int mss = 0;
>  #endif /* IXGBE_FCOE */
>  	u16 cleaned_count = ixgbe_desc_unused(rx_ring);
> +	struct sk_buff *skbs[BURST_SIZE];
> +	unsigned int skb_burst = 0;
> 
>  	while (likely(total_rx_packets < budget)) {
>  		union ixgbe_adv_rx_desc *rx_desc;
> @@ -2161,13 +2179,19 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
>  		}
>  
>  #endif /* IXGBE_FCOE */
> -		skb_mark_napi_id(skb, &q_vector->napi);
> -		ixgbe_rx_skb(q_vector, skb);
> -
>  		/* update budget accounting */
>  		total_rx_packets++;
> +		skbs[skb_burst++] = skb;
> +
> +		if (skb_burst == BURST_SIZE) {
> +			ixgbe_rx_skb_burst(skbs, skb_burst, q_vector,
> +					   rx_ring->netdev);
> +			skb_burst = 0;
> +		}
>  	}
>  
> +	ixgbe_rx_skb_burst(skbs, skb_burst, q_vector, rx_ring->netdev);
> +
>  	u64_stats_update_begin(&rx_ring->syncp);
>  	rx_ring->stats.packets += total_rx_packets;
>  	rx_ring->stats.bytes += total_rx_bytes;

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer