netdev - Re: [PATCH net-next 2/6] net: nexthop: Increase weight to u16

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Message-ID: <8734nj5esp.fsf@nvidia.com>
Date: Mon, 5 Aug 2024 14:33:45 +0200
From: Petr Machata <petrm@...dia.com>
To: Simon Horman <horms@...nel.org>
CC: Petr Machata <petrm@...dia.com>, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, "Paolo
 Abeni" <pabeni@...hat.com>, <netdev@...r.kernel.org>, Ido Schimmel
	<idosch@...dia.com>, David Ahern <dsahern@...nel.org>, Donald Sharp
	<sharpd@...dia.com>, <mlxsw@...dia.com>
Subject: Re: [PATCH net-next 2/6] net: nexthop: Increase weight to u16


Simon Horman <horms@...nel.org> writes:

> On Thu, Aug 01, 2024 at 06:23:58PM +0200, Petr Machata wrote:
>> In CLOS networks, as link failures occur at various points in the network,
>> ECMP weights of the involved nodes are adjusted to compensate. With high
>> fan-out of the involved nodes, and overall high number of nodes,
>> a (non-)ECMP weight ratio that we would like to configure does not fit into
>> 8 bits. Instead of, say, 255:254, we might like to configure something like
>> 1000:999. For these deployments, the 8-bit weight may not be enough.
>> 
>> To that end, in this patch increase the next hop weight from u8 to u16.
>> 
>> Increasing the width of an integral type can be tricky, because while the
>> code still compiles, the types may not check out anymore, and numerical
>> errors come up. To prevent this, the conversion was done in two steps.
>> First the type was changed from u8 to a single-member structure, which
>> invalidated all uses of the field. This allowed going through them one by
>> one and audit for type correctness. Then the structure was replaced with a
>> vanilla u16 again. This should ensure that no place was missed.
>> 
>> The UAPI for configuring nexthop group members is that an attribute
>> NHA_GROUP carries an array of struct nexthop_grp entries:
>> 
>> 	struct nexthop_grp {
>> 		__u32	id;	  /* nexthop id - must exist */
>> 		__u8	weight;   /* weight of this nexthop */
>> 		__u8	resvd1;
>> 		__u16	resvd2;
>> 	};
>> 
>> The field resvd1 is currently validated and required to be zero. We can
>> lift this requirement and carry high-order bits of the weight in the
>> reserved field:
>> 
>> 	struct nexthop_grp {
>> 		__u32	id;	  /* nexthop id - must exist */
>> 		__u8	weight;   /* weight of this nexthop */
>> 		__u8	weight_high;
>> 		__u16	resvd2;
>> 	};
>> 
>> Keeping the fields split this way was chosen in case an existing userspace
>> makes assumptions about the width of the weight field, and to sidestep any
>> endianes issues.
>
> nit: endianness

Thanks, will fix for v2. For now I'm still waiting if there's other
feedback.