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

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Message-ID: <5cca8bdd-bed0-f26a-6c96-d18947d3a50b@gmail.com>
Date:   Mon, 24 Jan 2022 11:29:52 +0200
From:   Julian Wiedmann <jwiedmann.dev@...il.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>, netdev@...r.kernel.org
Cc:     Shay Agroskin <shayagr@...zon.com>,
        Arthur Kiyanovski <akiyano@...zon.com>,
        David Arinzon <darinzon@...zon.com>,
        Noam Dagan <ndagan@...zon.com>,
        Saeed Bishara <saeedb@...zon.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Sameeh Jubran <sameehj@...zon.com>,
        Wei Yongjun <weiyongjun1@...wei.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: ena: Do not waste napi skb cache

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@...il.com>
>> ---
>>  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?

>>  	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,
>