netdev - Re: [PATCH bpf-next 4/4] ixgbe, xsk: use XSK_NAPI

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Date:   Tue, 8 Sep 2020 07:38:38 +0200
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Björn Töpel <bjorn.topel@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org, magnus.karlsson@...el.com,
        intel-wired-lan@...ts.osuosl.org
Subject: Re: [PATCH bpf-next 4/4] ixgbe, xsk: use XSK_NAPI_WEIGHT as NAPI poll
 budget

On 2020-09-07 21:32, Jakub Kicinski wrote:
> On Mon,  7 Sep 2020 17:02:17 +0200 Björn Töpel wrote:
>> From: Björn Töpel <bjorn.topel@...el.com>
>>
>> Start using XSK_NAPI_WEIGHT as NAPI poll budget for the AF_XDP Rx
>> zero-copy path.
>>
>> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
>> ---
>>   drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
>> index 3771857cf887..f32c1ba0d237 100644
>> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
>> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
>> @@ -239,7 +239,7 @@ int ixgbe_clean_rx_irq_zc(struct ixgbe_q_vector *q_vector,
>>   	bool failure = false;
>>   	struct sk_buff *skb;
>>   
>> -	while (likely(total_rx_packets < budget)) {
>> +	while (likely(total_rx_packets < XSK_NAPI_WEIGHT)) {
> 
> I was thinking that we'd multiply 'budget' here, not replace it with a
> constant. Looks like ixgbe dutifully passes 'per_ring_budget' into the
> clean_rx functions, not a complete NAPI budget.
>

Correct, and i40e/ice does the same ("per_ring_budget").

As for budget << XSK_NAPI_MULT vs replacing; Replacing the budget is 
more in line with what the drivers do for the Tx cleanup 
(xxx_clean_tx_irq), where the napi budget is discarded completely; 
Again, with the idea that "this is much cheaper than a "per-packet 
through the stack".

Do you prefer the multiplier way that you describe?


Cheers,
Björn


>>   		union ixgbe_adv_rx_desc *rx_desc;
>>   		struct ixgbe_rx_buffer *bi;
>>   		unsigned int size;
>