netdev - RE: [PATCH net-next] tcp: shrink tcp6_timewait

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Date:	Wed, 02 Oct 2013 07:57:10 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] tcp: shrink tcp6_timewait_sock by one cache
 line

On Wed, 2013-10-02 at 13:08 +0100, David Laight wrote:
> > -	tmo = tw->tw_ttd - jiffies;
> > +	tmo = tw->tw_ttd - (u32)jiffies;
> 
> Do you need any of these (u32) casts?
> The compiler will almost certainly use 32bit arithmetic (on 32bit systems at least)
> because the 'as if' rule lets if use the smaller type.

I wanted to clearly show the intent of the code.

Some compilers might warnings here.

> 
> > +		tw->tw_ttd = (u32)(jiffies + (slot << INET_TWDR_RECYCLE_TICK));
> 
> If that (u32) cast is needed in order to avoid 64bit maths, it is in the wrong place.

I want to make sure no compiler will complain for potential losses of
precision.

Note we have :

include/net/tcp.h:691:#define tcp_time_stamp            ((__u32)(jiffies))

But this could change if we want 100us resolution for TCP timestamps at
one point.

In this code, we want HZ units, but truncated to 32bits.

Adding a 

#define jiffies32   ((u32)jiffies)

might do the trick, but lot of pain for such a trivial patch.


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