On 7/22/16, 12:20 PM, Eric W. Biederman wrote:
> Roopa Prabhu <[email protected]> writes:
>
>> On 7/21/16, 1:00 PM, Eric W. Biederman wrote:
>>> Roopa Prabhu <[email protected]> writes:
>>>
>>>
[snip]
>>> I did not realize it is hardcoded to 8 in iproute2. Because kernel has a
>>> hard coded limit of
>>> 2.
>>> I think we need to fix it in a few places:
>>> a) we should move the kernel #define to a uapi header file which iproute2
>>> can use
>>> b) there has been a general ask to bump the kernel MAX_LABELS from 2 and I
>>> don't see
>>> a problem with it yet. so, we could bump it to 8.
>>>
>>> Were you planning to post patches for one or both of the above ?.
>>>
>>> I can post them too. Let me know.
>>> a) I just looked and the kernel netlink protocol does not have a limit.
>>> The kernel does have a limit but the netlink protocol does not so
>>> there is no point in exporting a limit in a uapi header, it will
>>> just be out of date and wrong.
>> sure, if you have concerns about making it part of uapi, we can
>> separately maintain the same limit in iproute2 and kernel.
> The different tools already have different limits and it is not
> a problem. The important thing is for the userspace tool to have
> the larger limit.
>>> b) I can see in principle bumping up the kernels MAX_LABELS past two
>>> although I haven't heard those requests, or understand the use cases.
>>> I don't recall seeing any ducumentation on cases where it is
>>> desirable to push a lot of labels at once. (Do hardware
>>> implementations support pushing a lot of labels at once?)
>> I don't know of any use cases either. But i have received multiple requests
>> on bumping the current limit of two
>>> Bumping past 8 seems quite a lot. That starts feeling like people
>>> trying to break other peoples mpls stacks. That is asking for more
>>> packet space for labels than ipv6 uses for addresses and ipv6 is way
>>> oversized. The commonly agreed wisdom is the world only needs 40 to
>>> 48 bits to route on to reach the entire world.
>>>
>>> I can completely understand a few specialty labels going beyond what
>>> is needed for general purpose routing but pushing more that 8 at
>>> once seems huge. Especially since you can recirculate packets if
>>> you really need to and push more labels that way.
>> I don't think there is an ask for going more than 8. anything greater than
>> current 2 is good.
> Except the patch that got all of this started.
ok, missed that. yesterday I also received some info on a segment routing
use-case
where there is an ongoing study which is currently leaning towards a max label
stack
depth of 17.
>
>>> Add to that for a software implementation we have these pesky things
>>> called cache lines. I can see in the kernel pushing struct
>>> mpls_route towards the size of a full cacheline. Today we are at 52
>>> bytes not counting the via adress. With the via address we are at 56
>>> (ipv4), 58 (ethernet), and 60 (ipv6) bytes. Which means in we have
>>> to make the kernel data structures smarter or we risk messing up the
>>> performance of the common case.
>>>
>>> Also we do need some kind of limit in the kernel to protect against
>>> insane inputs.
>>>
>>> So while I can imagine there are reasonable cases for bumping up the
>>> maximum number of labels in the kernel I think we need to be smart if
>>> we ware going to do that. Which probably means we will want a
>>> __mpls_nh_label helper function.
>>>
>> sure, yes, the current static label array works well for the common case
>> of 2 labels. does it make sense for it to be configurable
>> with the default being 2 and max something like 8 ?
> We have two structures both with one byte holes:
> struct mpls_route { /* next hop label forwarding entry */
> struct rcu_head rt_rcu;
> u8 rt_protocol;
> u8 rt_payload_type;
> u8 rt_max_alen;
> unsigned int rt_nhn;
> unsigned int rt_nhn_alive;
> struct mpls_nh rt_nh[0];
> };
>
> struct mpls_nh { /* next hop label forwarding entry */
> struct net_device __rcu *nh_dev;
> unsigned int nh_flags;
> u32 nh_label[MAX_NEW_LABELS];
> u8 nh_labels;
> u8 nh_via_alen;
> u8 nh_via_table;
> };
>
> If we were to define them as:
> struct mpls_route { /* next hop label forwarding entry */
> struct rcu_head rt_rcu;
> u8 rt_protocol;
> u8 rt_payload_type;
> u8 rt_max_alen;
> u8 rt_max_labels;
> unsigned int rt_nhn;
> unsigned int rt_nhn_alive;
> struct mpls_nh rt_nh[0];
> };
>
> struct mpls_nh { /* next hop label forwarding entry */
> struct net_device __rcu *nh_dev;
> unsigned int nh_flags;
> u8 nh_labels;
> u8 nh_via_alen;
> u8 nh_via_table;
> };
>
> static 32 *__mpls_nh_labels(struct mpls_route *rt, struct mpls_nh *nh)
> {
> u32 *nh0_labels = PTR_ALIGN((u32 *)&rt->rt_nh[rt->rt_nhn], sizeof(u32));
> int nh_index = nh - rt->rt_nh;
>
> return nh0_labels + rt->rt_max_labels * nh_index;
> }
>
> static u8 *__mpls_nh_via(struct mpls_route *rt, struct mpls_nh *nh)
> {
> u8 *nh0_via = PTR_ALIGN((u8 *)(&rt->rt_nh[rt->rt_nhn] + (sizeof(u32)
> *rt->max_labels * rt->nhn)), VIA_ALEN_ALIGN);
> int nh_index = nh - rt->rt_nh;
>
> return nh0_via + rt->rt_max_alen * nh_index;
> }
>
> Ugh. I just noticed we have a nasty 4 byte gap in the mpls_route by
> having both rt_nhn and rt_nhn_alive in there. As rt_nh[0] has pointer
> alignment.
>
> Anyway something like the above should allow us to remove the limit
> of the number of labels from the implementation and still fit everything
> in a cache line in the common case, as the change above doesn't take up
> any extra space in struct mpls_route.
>
> Then we just pick a reasonable maximum and set MAX_NEW_LABELS to that.
> That will change struct mpls_route_config. So we need a small enough
> value that putting struct mpls_route_config continues to make sense.
> I propose 8 for MAX_NEW_LABELS after such a change.
>
> It looks pretty straighforward on the kernel side.
I like it. It follows how via is handled today and I agree seems like the best
way to
represent varying number of labels without affecting the common case.
thanks for the suggestion.
