netdev - Re: [PATCH v2 net 2/2] Revert "net: skb: introduce and use a single page frag cache"

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Message-ID: <CANn89iLVy7d8m0KAEHprhv2y2QjdLcYAHCZxYmg8RbyiZEYvZA@mail.gmail.com>
Date: Thu, 20 Feb 2025 10:41:57 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
	Alexander Duyck <alexanderduyck@...com>, Nikolay Aleksandrov <razor@...ckwall.org>, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH v2 net 2/2] Revert "net: skb: introduce and use a single
 page frag cache"

On Tue, Feb 18, 2025 at 7:33 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> After the previous commit is finally safe to revert commit dbae2b062824
> ("net: skb: introduce and use a single page frag cache"): do it here.
>
> The intended goal of such change was to counter a performance regression
> introduced by commit 3226b158e67c ("net: avoid 32 x truesize
> under-estimation for tiny skbs").
>
> Unfortunately, the blamed commit introduces another regression for the
> virtio_net driver. Such a driver calls napi_alloc_skb() with a tiny
> size, so that the whole head frag could fit a 512-byte block.
>
> The single page frag cache uses a 1K fragment for such allocation, and
> the additional overhead, under small UDP packets flood, makes the page
> allocator a bottleneck.
>
> Thanks to commit bf9f1baa279f ("net: add dedicated kmem_cache for
> typical/small skb->head"), this revert does not re-introduce the
> original regression. Actually, in the relevant test on top of this
> revert, I measure a small but noticeable positive delta, just above
> noise level.
>
> The revert itself required some additional mangling due to recent updates
> in the affected code.
>
> Suggested-by: Eric Dumazet <edumazet@...gle.com>
> Fixes: dbae2b062824 ("net: skb: introduce and use a single page frag cache")
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>