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

Andy Gospodarek
2015-05-28T22:35:32Z

On Thu, May 28, 2015 at 08:40:11AM -0700, Scott Feldman wrote:
> On Thu, May 28, 2015 at 2:42 AM, Jiri Pirko <jiri@resnulli.us> wrote:
> > Mon, May 18, 2015 at 10:19:16PM CEST, davem@davemloft.net wrote:
> >>From: Roopa Prabhu <roopa@cumulusnetworks.com>
> >>Date: Sun, 17 May 2015 16:42:05 -0700
> >>
> >>> On most systems where you can offload routes to hardware,
> >>> doing routing in software is not an option (the cpu limitations
> >>> make routing impossible in software).
> >>
> >>You absolutely do not get to determine this policy, none of us
> >>do.
> >>
> >>What matters is that by default the damn switch device being there
> >>is %100 transparent to the user.
> >>
> >>And the way to achieve that default is to do software routes as
> >>a fallback.
> >>
> >>I am not going to entertain changes of this nature which fail
> >>route loading by default just because we've exceeded a device's
> >>HW capacity to offload.
> >>
> >>I thought I was _really_ clear about this at netdev 0.1
> >
> > I certainly agree that by default, transparency 1:1 sw:hw mapping is
> > what we need for fib. The current code is a good start!
> >
> > I see couple of issues regarding switchdev_fib_ipv4_abort:
> > 1) If user adds and entry, switchdev_fib_ipv4_add fails, abort is
> > executed -> and, error returned. I would expect that route entry should
> > be added in this case. The next attempt of adding the same entry will
> > be successful.
> > The current behaviour breaks the transparency you are reffering to.
> > 2) When switchdev_fib_ipv4_abort happens to be executed, the offload is
> > disabled for good (until reboot). That is certainly not nice, alhough
> > I understand that is the easiest solution for now.
> >
> > I believe that we all agree that the 1:1 transparency, although it is a
> > default, may not be optimal for real-life usage. HW resources are
> > limited and user does not know them. The danger of hitting _abort and
> > screwing-up the whole system is huge, unacceptable.
> >
> > So here, there are couple of more or less simple things that I suggest to
> > do in order to move a little bit forward:
> > 1) Introduce system-wide option to switch _abort to just plain fail.
> > When HW does not have capacity, do not flush and fallback to sw, but
> > rather just fail to add the entry. This would not break anything.
> > Userspace has to be prepared that entry add could fail.
> > 2) Introduce a way to propagate resources to userspace. Driver knows about
> > resources used/available/potentially_available. Switchdev infra could
> > be extended in order to propagate the info to the user.
> > 3) Introduce couple of flags for entry add that would alter the default
> > behaviour. Something like:
> > NLM_F_SKIP_KERNEL
> > NLM_F_SKIP_OFFLOAD
> > Again, this does not break the current users. On the other hand, this
> > gives new users a leverage to instruct kernel where the entry should
> > be added to (or not added to).
> >
> > Any thoughts? Objections?
>
> I don't like these. Breaks transparency and forces the user in a
> position of having to know hardware failures modes (unique to each
> hardware device). I presented an option d) which avoids this issues;
> was it not understood?

I actually really like the way Jiri succinctly covered the different
cases to move us forward from what we have today (Thanks, Jiri!). I
completely agree with you on both of your problem statements and the
idea that what have is fine for the short-term. I see definite room to
improve the the user experience available via upstream kernels.

Option 1 has appeal since userspace applications that control FDB, FIB,
etc entries could work without modification (the when in this mode the
kernel could choose to ignore any NLM_F_* flags Jiri proposed), but I
agree that a system-wide (or maybe offload-device-wide?) configuration
option needs to exist as this should not be the default behavior.

Option 2 could also work as userspace applications could query for
space availability before attempting to add a route. This could be
nice during bootup as then apps could periodically double check that
their view of the world is accurate.

Option 3 also has appeal since there exists the ability to allow
fine-grained control from userspace applications since less used routes
(or routes that could be summarized) could be combined in userspace if
needed.

The great part about all suggestions is that when combined they can
provide a great user experience, but doing all 3 at once is probably too
aggressive. My vote would be to see if we can work together on a
combination of Option 1 and 3 together as they seem to provide a great
first start to this...

If an application tried to add a route (called A) to the route table
in the kernel and code to support Option 1 existed (similar to what
Roopa posted to start this series) then the kernel could fail to add
route A.

If the user noted that some other route (called B) was lower priority
for _any_ reason, the user could delete route B from the kernel and
hardware and add route A to hardware and kernel. Then the user could
make a call to add route B with the flag 'NLM_F_SKIP_OFFLOAD' which
would tell the kernel not to perform a FIB offload of that route.

Now we have some routes in the table that will be offloaded to hardware
and software and some that will be handled only in software -- as long
as the user has explicitly enabled some sort of offload option.

The lingering question in my mind is, what interface do we use to
configure it....