netdev - Re: [PATCH net-next] net_sched: add 64bit rate estimators

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Date:	Wed, 5 Jun 2013 21:06:09 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net_sched: add 64bit rate estimators

On Wed, 2013-06-05 at 12:37 -0700, Eric Dumazet wrote:
> On Wed, 2013-06-05 at 19:55 +0100, Ben Hutchings wrote:
> > On Tue, 2013-06-04 at 22:00 -0700, Eric Dumazet wrote:
> > > From: Eric Dumazet <edumazet@...gle.com>
> > > 
> > > struct gnet_stats_rate_est contains u32 fields, so the byte per second
> > > field can wrap at 34360Mbit.
> > > 
> > > Add a new gnet_stats_rate_est64 structure to get 64bit bps field, and
> > > switch the kernel to use this structure natively.
> > [...] 
> > >  /**
> > > + * struct gnet_stats_rate_est64 - rate estimator
> > > + * @bps: current byte rate
> > > + * @pps: current packet rate
> > > + */
> > > +struct gnet_stats_rate_est64 {
> > > +	__u64	bps;
> > > +	__u32	pps;
> > > +} __attribute__ ((packed));
> > [...]
> > 
> > Using attribute packed (which should be written as __packed) forces the
> > alignment of each member and the structure as a whole to be 1.  For an
> > architecture where unaligned access is expensive, gcc will make all
> > reads and writes use byte operations.
> > 
> > I think you have to add a dummy __u32 member instead.
> 
> Its exported to userland so __packed is not available. Or has this been
> changed lately ?

Sorry, you're quite right.

> This structure is only used in very slow path, in the rate estimator
> thing, so the unaligned accesses are not a problem at all.
> 
> I made this choice to not increase sizeof(struct Qdisc) and the
> alignments as well.
> 
> If you think there is an issue with userland, we could use a
> gnet_stats_rate_est64_packed in kernel, and a gnet_stats_rate_est64 in
> userland.

If you're sure this is the right trade-off then I accept that.  I
thought you were using packed to ensure the structure layout was
consistent between userland and kernel, and might not have realised the
unfortunate effect on alignment.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
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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