netdev - Re: [PATCH net-next-2.6] filter: optimize sk_run

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Date:	Fri, 19 Nov 2010 20:32:24 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Hagen Paul Pfeifer <hagen@...u.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] filter: optimize sk_run_filter

On Fri, Nov 19, 2010 at 7:17 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le vendredi 19 novembre 2010 à 10:54 +0100, Eric Dumazet a écrit :
>
>> I believe we should revert the u32 f_k = fentry->k; part
>>
>> fentry->k as is fast as f_k if stored on stack, and avoids one
>> instruction if fentry->k is not needed.
>>
>>
>
> A revert is not good on arches with decent number of registers (x86_64
> for example).
>
> Maybe have some CONFIG_ARCH_HAS_{FEW|MANY}_REGISTERS is needed, (or
> already exist ?)
>
> Here is the patch to save 400 bytes on x86_32, and really speedup the
> damn thing on all arches.
>
> Thanks
>
> [PATCH net-next-2.6] filter: optimize sk_run_filter
>
> remove pc variable to avoid arithmetic to compute fentry at each filter
> instruction. Jumps directly manipulate fentry pointer.
>
> As the last instruction of filter[] is guaranteed to be a RETURN, and
> all jumps are before the last instruction, we dont need to check filter
> bounds (number of instructions in filter array) at each iteration, so we
> remove it from sk_run_filter() params.
>
> On x86_32 remove f_k var introduced in commit 57fe93b374a6b871
> (filter: make sure filters dont read uninitialized memory)
>
> Note : We could use a CONFIG_ARCH_HAS_{FEW|MANY}_REGISTERS in order to
> avoid too many ifdefs in this code.
>
> This helps compiler to use cpu registers to hold fentry and A
> accumulator.
>
> On x86_32, this saves 401 bytes, and more important, sk_run_filter()
> runs much faster because less register pressure (One less conditional
> branch per BPF instruction)
>
> # size net/core/filter.o net/core/filter_pre.o
>   text    data     bss     dec     hex filename
>   2948       0       0    2948     b84 net/core/filter.o
>   3349       0       0    3349     d15 net/core/filter_pre.o
>
> on x86_64 :
> # size net/core/filter.o net/core/filter_pre.o
>   text    data     bss     dec     hex filename
>   5173       0       0    5173    1435 net/core/filter.o
>   5224       0       0    5224    1468 net/core/filter_pre.o
>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> Cc: Changli Gao <xiaosuo@...il.com>
> Cc: Hagen Paul Pfeifer <hagen@...u.net>
> ---
>  include/linux/filter.h  |    2
>  net/core/filter.c       |   93 +++++++++++++++++++-------------------
>  net/core/timestamping.c |    2
>  net/packet/af_packet.c  |    2
>  4 files changed, 51 insertions(+), 48 deletions(-)
>

you missed the users of sk_run_filter in directory dev/.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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