netdev - Re: [PATCH] macvlan: lockless tx path

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Date:	Wed, 10 Nov 2010 21:04:16 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ben Greear <greearb@...delatech.com>,
	David Miller <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] macvlan: lockless tx path

On Wed, 2010-11-10 at 21:33 +0100, Eric Dumazet wrote:
> Le mercredi 10 novembre 2010 à 10:40 -0800, Ben Greear a écrit :
> 
> > In my opinion, the kernel and/or driver should deal with this such that
> > at worst the user has to deal with 32 v/s 64 bits based on whether the
> > kernel is compiled for 32 or 64 bit CPUs.  (Let the driver sample at
> > intervals needed to never wrap it's counters more than once and update
> > software stats of well-defined bit-width, and present those software
> > counters to users.
> > 
> 
> How so ? Are you willing to provide patches for all network drivers ?
> 
> > In practice, this seems to be the case, at least for the NICs I've used
> > (mostly Intel).  But, please don't propagate the idea that any width of
> > counters is OK to present to user-space:  It is completely unfair to
> > make app writers have to know the network driver and/or hardware quirks to
> > know how often it must sample stats.
> > 
> 
> I am sorry Ben, but /proc/net/dev doesnt publish each counter effective
> width. Its unfair, but its like that.
> 
> An appplication must be able to cope for wrap arounds, running on a 32
> or 64bit kernel. Our duty is to provide 64bit counters for high speed
> interfaces where possible.
> For a 10Mb adapter, there is no need, since a 32bit counter doesnt wrap
> in less than one hour (RFC1902 suggestion)
> 
> As I said, many drivers counters are not 32bit or 64bit. I did many
> driver get_stats() checks lately...
> 
> Why should we cap them to 32bit if they really are 36 or 40 bits ? 
[...]

Drivers should calculate differences and accumulate them in a 64-bit
counter.  (A lot of hardware has read-to-clear counters anyway, in which
case the driver *has* to accumulate the values it reads.)

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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