netdev - Re: [PATCH] net: bpf_jit: Simplify code by always using offset8 or offset32.

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Date:	Tue, 20 Mar 2012 22:33:35 +1100
From:	"Indan Zupancic" <indan@....nu>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	"Will Drewry" <wad@...omium.org>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org,
	x86@...nel.org, arnd@...db.de, davem@...emloft.net, hpa@...or.com,
	mingo@...hat.com, oleg@...hat.com, peterz@...radead.org,
	rdunlap@...otime.net, mcgrathr@...omium.org, tglx@...utronix.de,
	luto@....edu, eparis@...hat.com, serge.hallyn@...onical.com,
	djm@...drot.org, scarybeasts@...il.com, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, markus@...omium.org,
	coreyb@...ux.vnet.ibm.com, keescook@...omium.org
Subject: Re: [PATCH] net: bpf_jit: Simplify code by always using offset8 or
 offset32.

On Tue, March 20, 2012 13:59, Eric Dumazet wrote:
> On Tue, 2012-03-20 at 13:24 +1100, Indan Zupancic wrote:
>
>> If it does then perhaps the fast path should be made faster by inlining
>> the code instead of calling a function which may not be cached.
>>
>
> inlining 400 times a sequence of code is waste of icache, you probably
> missed this.

Well, according to you most filters were small, inling 26 bytes a few
times should be faster than calling an external function. Not all calls
need to be inlined either.

>
> I spent a lot of time on working on this implementation, tried many
> different strategies before choosing the one in place.
>
> Listen, I am tired of this thread, it seems you want to push changes
> that have almost no value but still need lot of review.

The latest patch didn't change generated code except for a few ancillary
instructions. The one before that just added documentation. The first
patch was indeed bad.

>
> Unless you make benchmarks and can make at least 5 % improvement of the
> speed, or improve maintainability of this code, I am not interested.

My next patch would have changed the compiler to always compile in two
passes instead of looping till the result is stable. But never mind.

>
> We certainly _can_ one day have sizeof(struct sk_buff) > 256, and actual
> code is ready for this. You want to break this for absolutely no valid
> reason.

I've the feeling you didn't read the latest patch, it doesn't assume
sizeof(struct sk_buff) < 256, nor that fields aren't reordered.

>
> We _can_ change fields order anytime in struct sk_buff, even if you
> state "its very unlikely that those fields are ever moved to the end
> of sk_buff".

And if the dev, len or data_len fields are really moved past the first
127 bytes the JIT code can be changed too. The JIT code already depends
on some of struct sk_buff's field properties anyway.

Greetings,

Indan


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