Subject: Re: [PATCH net v2] switchdev: don't abort hardware ipv4 fib offload on failure to program fib entry in hardware

From: roopa <[email-masked]>
Date: Thu, 21 May 2015 08:37:30 -0700

On 5/20/15, 10:46 PM, Scott Feldman wrote:
> On Tue, May 19, 2015 at 1:28 PM, David Miller <[email-masked]> wrote:
>> From: Andy Gospodarek <[email-masked]>
>> Date: Tue, 19 May 2015 15:47:32 -0400
>>
>>> Are you actually saying that if users complain loudly enough about
>>> the current behavior (not the change Roopa has proposed) that you
>>> would be open to considering a change the current behavior?
>> I am saying that we have a contract with users not to break existing
>> behavior.  Full stop.
> After rehearing David's argument, we should probably explore option d)
> which is a refinement on the fib_offload_disable mechanism we have
> today.  fib_offload_disable is global for all routes.  Once we hit a
> HW install problem, the global flag is set and all routes fallback to
> SW.  We did this because we can't allow the failed route to exist in
> SW and not in HW because it could mess up LPM searches (HW could hit
> on a lesser prefix even when SW has the true LPM, because HW gets
> first shot at match).  The refinement on fib_offload_disable is this:
> make it per-related-prefix rather than global, and on a HW install
> problem, set the flag for the related-prefix and uninstall only those
> routes from HW.  Related-prefix (is there a correct term for this?)
> are routes to the same dst addr but with different prefix lengths.  I
> haven't parsed the fib_trie structure to see how routes are organized,
> but I suspect since it's optimized for lookup the related-prefix
> tracking is already there and we can build on that.
>
> Option d) requires no application changes.  It requires no additional
> understanding or input from the user.  Kernel fallback happens
> transparently.  In the case where the HW install failure was due to
> out-of-resource in HW, there may be some oscillation as
> related-prefixes are removed from HW, freeing up a little space, only
> to be filled as new routes come in, and so on.  Actually, now that I
> think of it, the device/driver could decide which related-prefix to
> evict from HW, if driver/device wanted to have a sense of which routes
> are more important to offload than others, when resources are limited.
>
> I think the parts we need are:
>
> 1) A new fib_offload_disable bit for related-prefixes.
> 2) On switchdev fib offload, look up if route is marked as
> fib_offload_disabled based on it's related-prefix membership
> 3) A notification mechanism from driver to indicate a related-prefix
> is fib_offload_disabled.
>
> Feasible?
neat idea scott. But in practice I would be a bit nervous about getting 
the distribution of related prefixes between
hardware and software right (need to think about the nuances a bit more).

I am still leaning towards the possibility of making this a global 
system policy decision and leave the app and the switch driver less 
surprised. But this needs a better explicit offload policy/resource 
manager infra. I don't have any concrete thoughts on it yet.
(Understand that it will also need to make sure it does not break the 
existing contract with Linux apps (that dave was pointing too)).

thanks.