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Paolo Abeni 6bc7e4eb04 Revert "net: skb: introduce and use a single page frag cache" 
After the previous commit is finally safe to revert commit dbae2b0628
("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 3226b158e6 ("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 bf9f1baa27 ("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@google.com>
Fixes: dbae2b0628 ("net: skb: introduce and use a single page frag cache")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
History 2025-02-20 10:53:25 +01:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.