netdev - Re: [PATCH v2 net] tcp: Fix integer-overflows in TCP vegas

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Date:	Sat, 26 Jul 2014 11:54:57 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Paasch <christoph.paasch@...ouvain.be>
Cc:	Stephen Hemminger <stephen@...workplumber.org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Neal Cardwell <ncardwell@...gle.com>,
	David Laight <David.Laight@...LAB.COM>,
	Doug Leith <doug.leith@...m.ie>
Subject: Re: [PATCH v2 net] tcp: Fix integer-overflows in TCP vegas

On Sat, 2014-07-26 at 10:59 +0200, Christoph Paasch wrote:

> do you mean, using "do_div"?
> 
> David suggested to avoid using do_div in tcp_vegas.

My understanding is the following :

On 64bit arches, used on most servers that really care of TCP
performance these days, do_div() is the fastest thing : No extra
conditional.

# define do_div(n,base) ({                                      \
        uint32_t __base = (base);                               \
        uint32_t __rem;                                         \
        __rem = ((uint64_t)(n)) % __base;                       \
        (n) = ((uint64_t)(n)) / __base;                         \
        __rem;                                                  \
 })


Then on 32bit, do_div(target_cwnd, Y) will perform a single divide
if target_cwnd is < 2^32, which is very likely the case :


# define do_div(n,base) ({                              \
        uint32_t __base = (base);                       \
        uint32_t __rem;                                 \
        (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
        if (likely(((n) >> 32) == 0)) {                 \
                __rem = (uint32_t)(n) % __base;         \
                (n) = (uint32_t)(n) / __base;           \
        } else                                          \
                __rem = __div64_32(&(n), __base);       \
        __rem;                                          \
 })



(In both cases, compiler will remove the modulo operation, as we do not use it)



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