Re: [PATCH] net: ena: Do not waste napi skb cache

[Julian Wiedmann]

On 24.01.22 22:50, Shay Agroskin wrote:
> 
> Julian Wiedmann <jwiedmann.dev@xxxxxxxxx> writes:
> 
>> On 24.01.22 10:57, Julian Wiedmann wrote:
>>> On 23.01.22 13:56, Hyeonggon Yoo wrote:
>>>> By profiling, discovered that ena device driver allocates skb by
>>>> build_skb() and frees by napi_skb_cache_put(). Because the driver
>>>> does not use napi skb cache in allocation path, napi skb cache is
>>>> periodically filled and flushed. This is waste of napi skb cache.
>>>>
>>>> As ena_alloc_skb() is called only in napi, Use napi_build_skb()
>>>> instead of build_skb() to when allocating skb.
>>>>
>>>> This patch was tested on aws a1.metal instance.
>>>>
>>>> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
>>>> ---
>>>>  drivers/net/ethernet/amazon/ena/ena_netdev.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.c b/drivers/net/ethernet/amazon/ena/ena_netdev.c
>>>> index c72f0c7ff4aa..2c67fb1703c5 100644
>>>> --- a/drivers/net/ethernet/amazon/ena/ena_netdev.c
>>>> +++ b/drivers/net/ethernet/amazon/ena/ena_netdev.c
>>>> @@ -1407,7 +1407,7 @@ static struct sk_buff *ena_alloc_skb(struct ena_ring *rx_ring, void *first_frag)
>>>>          skb = netdev_alloc_skb_ip_align(rx_ring->netdev,
>>>>                          rx_ring->rx_copybreak);
>>>
>>> To keep things consistent, this should then also be napi_alloc_skb().
>>>
>>
>> And on closer look, this copybreak path also looks buggy. If rx_copybreak
>> gets reduced _while_ receiving a frame, the allocated skb can end up too
>> small to take all the data.
>>
>> @ ena maintainers: can you please fix this?
>>
> 
> Updating the copybreak value is done through ena_ethtool.c (ena_set_tunable()) which updates `adapter->rx_copybreak`.
> The adapter->rx_copybreak value is "propagated back" to the ring local attributes (rx_ring->rx_copybreak) only after an interface toggle which stops the napi routine first.
> 

That's unfortunate. ena_get_tunable() returns the updated adapter->rx_copybreak, how would a user know that their update isn't actually live yet?

> Unless I'm missing something here I don't think the bug you're describing exists.
> 

ack, thanks for double-checking!

> I agree that the netdev_alloc_skb_ip_align() can become napi_alloc_skb(). Hyeonggon Yoo, can you please apply this change as well to this patch?
> 
> Thanks,
> Shay
> 
> 
>>>>      else
>>>> -        skb = build_skb(first_frag, ENA_PAGE_SIZE);
>>>> +        skb = napi_build_skb(first_frag, ENA_PAGE_SIZE);
>>>>  
>>>>      if (unlikely(!skb)) {
>>>>          ena_increase_stat(&rx_ring->rx_stats.skb_alloc_fail,  1,
>>>