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

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Date:	Thu, 6 Jun 2013 09:39:59 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Ben Hutchings" <bhutchings@...arflare.com>,
	"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

> > 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.

Or mark the __u64 with (IIRC) __attribute__((aligned,4))
so that gcc will generate two 32bit accesses rather than
the eight byte accesses is the structure is packed.

	David