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

[Eric Dumazet <eric.dumazet@gmail.com>]

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@google.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 ?

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.