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

From: Alexander Duyck <[email-masked]>
Date: Fri, 03 Oct 2014 07:40:27 -0700

On 10/02/2014 12:36 AM, Jesper Dangaard Brouer wrote:
> 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 ;-)

Yes, but please keep in mind that multi-core is the more common use case
for many systems.

>> 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.).

To that end we may want to look to something like GRO to do the
buffering on the Rx side so that we could make use of GRO/GSO to send
blocks of buffers instead of one at a time.

>> 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! :-)

>>From my past experience this is very platform dependant.  For example
with DDIO or DCA features enabled on a system the memcpy is very cheap
since it is already in the cache.  It is one of the reasons for choosing
that as a means of working around the fact that we cannot use build_skb
and page reuse in the same driver.

> 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().

The other big item that I think many are overlooking is the cost of
mapping the buffer.  On systems with an IOMMU or even in some cases just
the swiotlb can add some significant cost to allocating a new buffer.

>> 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! :-)

One thought I had at one point was to try and add a flag to the DMA api
to indicate if the DMA api is trivial resulting in just a call to
virt_to_phys.  It might be worthwhile to look into something like that,
then we could split the receive processing into one of two paths, one
for non-trivial DMA mapping APIs, and one for trivial DMA mapping APIs
such as swiotlb on a device that supports all the memory in the system.

>> 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().

The problem is build_skb usage comes at a certain cost.  Specifically in
the case of small packets it can result in a larger memory footprint
since you cannot just reuse the same region in the buffer.  I suspect we
may need to look into some sort of compromise between build_skb and a
copybreak scheme for best cache performance on Xeon for example.

>> ---
>>  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;
>

For the burst size logic you might want to explore handling the
descriptors in 4 descriptor aligned chunks that should give you the best
possible performance since that would mean processing the descriptor
ring one cache-line at a time.

Thanks,

Alex