netdev - Re: [PATCH bpf-next 06/13] xsk: optimize for aligned case

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Date:   Tue, 28 Sep 2021 16:15:21 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     magnus.karlsson@...el.com, bjorn@...nel.org, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        maciej.fijalkowski@...el.com, ciara.loftus@...el.com,
        jonathan.lemon@...il.com, bpf@...r.kernel.org,
        anthony.l.nguyen@...el.com, llvm@...ts.linux.dev
Subject: Re: [PATCH bpf-next 06/13] xsk: optimize for aligned case

On Wed, Sep 22, 2021 at 09:56:06AM +0200, Magnus Karlsson wrote:
> From: Magnus Karlsson <magnus.karlsson@...el.com>
> 
> Optimize for the aligned case by precomputing the parameter values of
> the xdp_buff_xsk and xdp_buff structures in the heads array. We can do
> this as the heads array size is equal to the number of chunks in the
> umem for the aligned case. Then every entry in this array will reflect
> a certain chunk/frame and can therefore be prepopulated with the
> correct values and we can drop the use of the free_heads stack. Note
> that it is not possible to allocate more buffers than what has been
> allocated in the aligned case since each chunk can only contain a
> single buffer.
> 
> We can unfortunately not do this in the unaligned case as one chunk
> might contain multiple buffers. In this case, we keep the old scheme
> of populating a heads entry every time it is used and using
> the free_heads stack.
> 
> Also move xp_release() and xp_get_handle() to xsk_buff_pool.h. They
> were for some reason in xsk.c even though they are buffer pool
> operations.
> 
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>

My apologies if this has already been reported (I have not seen a report
on netdev nor a report from Intel around it) but this patch as
commit 94033cd8e73b ("xsk: Optimize for aligned case") in -next causes
the following build failure with clang + x86_64 allmodconfig:

net/xdp/xsk_buff_pool.c:465:15: error: variable 'xskb' is uninitialized when used here [-Werror,-Wuninitialized]
                        xp_release(xskb);
                                   ^~~~
net/xdp/xsk_buff_pool.c:455:27: note: initialize the variable 'xskb' to silence this warning
        struct xdp_buff_xsk *xskb;
                                 ^
                                  = NULL
1 error generated.

Cheers,
Nathan