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

[Shay Agroskin]

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.

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

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,