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

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Date:	Wed, 5 Jun 2013 19:55:31 +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 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.

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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