Re: [ovs-dev] [RFC PATCH 0/7] net: openvswitch: Reduce stack usage

[Aaron Conole <aconole@redhat.com>]

Ilya Maximets <i.maximets@ovn.org> writes:

> On 9/29/23 09:06, Nicholas Piggin wrote:
>> On Wed Sep 27, 2023 at 6:36 PM AEST, Ilya Maximets wrote:
>>> On 9/27/23 02:13, Nicholas Piggin wrote:
>>>> Hi,
>>>>
>>>> We've got a report of a stack overflow on ppc64le with a 16kB kernel
>>>> stack. Openvswitch is just one of many things in the stack, but it
>>>> does cause recursion and contributes to some usage.
>>>>
>>>> Here are a few patches for reducing stack overhead. I don't know the
>>>> code well so consider them just ideas. GFP_ATOMIC allocations
>>>> introduced in a couple of places might be controversial, but there
>>>> is still some savings to be had if you skip those.
>>>>
>>>> Here is one place detected where the stack reaches >14kB before
>>>> overflowing a little later. I massaged the output so it just shows
>>>> the stack frame address on the left.
>>>
>>> Hi, Nicholas.  Thanks for the patches!
>>>
>>> Though it looks like OVS is not really playing a huge role in the
>>> stack trace below.  How much of the stack does the patch set save
>>> in total?  How much patches 2-7 contribute (I posted a patch similar
>>> to the first one last week, so we may not count it)?
>> 
>> Stack usage was tested for the same path (this is backported to
>> RHEL9 kernel), and saving was 2080 bytes for that. It's enough
>> to get us out of trouble. But if it was a config that caused more
>> recursions then it might still be a problem.
>
> The 2K total value likely means that only patches 1 and 4 actually
> contribute much into the savings.  And I agree that running at
> 85%+ stack utilization seems risky.  It can likely be overflowed
> by just a few more recirculations in OVS pipeline or traversing
> one more network namespace on a way out.  And it's possible that
> some of the traffic will take such a route in your system even if
> you didn't see it yet.
>
>>> Also, most of the changes introduced here has a real chance to
>>> noticeably impact performance.  Did you run any performance tests
>>> with this to assess the impact?
>> 
>> Some numbers were posted by Aaron as you would see. 2-4% for that
>> patch, but I suspect the rest should have much smaller impact.
>
> They also seem to have a very small impact on the stack usage,
> so may be not worth touching at all, since performance evaluation
> for them will be necessary before they can be accepted.

Actually, it's also important to keep in mind that the vport_receive is
only happening once in my performance test.  I expect it gets worse when
running in the scenario (br-ex, br-int, br-tun setup).

>> 
>> Maybe patch 2 if you were doing a lot of push_nsh operations, but
>> that might be less important since it's out of the recursive path.
>
> It's also unlikely that you have NHS pipeline configured in OVS.
>
>> 
>>>
>>> One last thing is that at least some of the patches seem to change
>>> non-inlined non-recursive functions.  Seems unnecessary.
>>>
>>> Best regards, Ilya Maximets.
>>>
>> 
>> One thing I do notice in the trace:
>> 
>> 
>> clone_execute is an action which can be deferred AFAIKS, but it is
>> not deferred until several recursions deep.
>> 
>> If we deferred always when possible, then might avoid such a big
>> stack (at least for this config). Is it very costly to defer? Would
>> it help here, or is it just going to process it right away and
>> cause basically the same call chain?
>
> It may save at most two stack frames maybe, because deferred actions
> will be called just one function above in ovs_execute_actions(), and
> it will not save us from packets exiting openvswitch module and
> re-entering from a different port, which is a case in the provided
> trace.

It used to always be deferred but, IIUC we were hitting deferred actions
limit quite a bit so when the sample and clone actions were unified there
was a choice to recurse to avoid dropping packets.

> Also, I'd vote against deferring, because then we'll start hitting
> the limit of deferred actions much faster causing packet drops, which
> is already a problem for some OVN deployments.  And deferring involves
> copying a lot of memory, which will hit performance once again.
>
> Best regards, Ilya Maximets.