netdev - Re: [PATCH net-next v3 5/8] net: skbuff: introduce napi_skb_cache_get

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Message-ID: <877c6toay1.fsf@toke.dk>
Date: Fri, 17 Jan 2025 13:56:54 +0100
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>, Andrew Lunn
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>, Lorenzo Bianconi
 <lorenzo@...nel.org>, Daniel Xu <dxu@...uu.xyz>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
 <andrii@...nel.org>, John Fastabend <john.fastabend@...il.com>, Jesper
 Dangaard Brouer <hawk@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, netdev@...r.kernel.org, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 5/8] net: skbuff: introduce
 napi_skb_cache_get_bulk()

Alexander Lobakin <aleksander.lobakin@...el.com> writes:

> Add a function to get an array of skbs from the NAPI percpu cache.
> It's supposed to be a drop-in replacement for
> kmem_cache_alloc_bulk(skbuff_head_cache, GFP_ATOMIC) and
> xdp_alloc_skb_bulk(GFP_ATOMIC). The difference (apart from the
> requirement to call it only from the BH) is that it tries to use
> as many NAPI cache entries for skbs as possible, and allocate new
> ones only if needed.
>
> The logic is as follows:
>
> * there is enough skbs in the cache: decache them and return to the
>   caller;
> * not enough: try refilling the cache first. If there is now enough
>   skbs, return;
> * still not enough: try allocating skbs directly to the output array
>   with %GFP_ZERO, maybe we'll be able to get some. If there's now
>   enough, return;
> * still not enough: return as many as we were able to obtain.
>
> Most of times, if called from the NAPI polling loop, the first one will
> be true, sometimes (rarely) the second one. The third and the fourth --
> only under heavy memory pressure.
> It can save significant amounts of CPU cycles if there are GRO cycles
> and/or Tx completion cycles (anything that descends to
> napi_skb_cache_put()) happening on this CPU.
>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> Tested-by: Daniel Xu <dxu@...uu.xyz>

Neat idea!

Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>