Re: [ovs-dev] [RFC PATCH 4/7] net: openvswitch: ovs_vport_receive reduce stack usage

["Nicholas Piggin" <npiggin@gmail.com>]

On Fri Sep 29, 2023 at 6:38 PM AEST, Eelco Chaudron wrote:
>
>
> On 29 Sep 2023, at 9:00, Nicholas Piggin wrote:
>
> > On Fri Sep 29, 2023 at 1:26 AM AEST, Aaron Conole wrote:
> >> Nicholas Piggin <npiggin@gmail.com> writes:
> >>
> >>> Dynamically allocating the sw_flow_key reduces stack usage of
> >>> ovs_vport_receive from 544 bytes to 64 bytes at the cost of
> >>> another GFP_ATOMIC allocation in the receive path.
> >>>
> >>> XXX: is this a problem with memory reserves if ovs is in a
> >>> memory reclaim path, or since we have a skb allocated, is it
> >>> okay to use some GFP_ATOMIC reserves?
> >>>
> >>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> >>> ---
> >>
> >> This represents a fairly large performance hit.  Just my own quick
> >> testing on a system using two netns, iperf3, and simple forwarding rules
> >> shows between 2.5% and 4% performance reduction on x86-64.  Note that it
> >> is a simple case, and doesn't involve a more involved scenario like
> >> multiple bridges, tunnels, and internal ports.  I suspect such cases
> >> will see even bigger hit.
> >>
> >> I don't know the impact of the other changes, but just an FYI that the
> >> performance impact of this change is extremely noticeable on x86
> >> platform.
> >
> > Thanks for the numbers. This patch is probably the biggest perf cost,
> > but unfortunately it's also about the biggest saving. I might have an
> > idea to improve it.
>
> Also, were you able to figure out why we do not see this problem on x86 and arm64? Is the stack usage so much larger, or is there some other root cause? Is there a simple replicator, as this might help you profile the differences between the architectures?

I found some snippets of equivalent call chain (this is for 4.18 RHEL8
kernels, but it's just to give a general idea of stack overhead
differences in C code). Frame size annotated on the right hand side:

[c0000007ffdba980] do_execute_actions     496
[c0000007ffdbab70] ovs_execute_actions    128
[c0000007ffdbabf0] ovs_dp_process_packet  208
[c0000007ffdbacc0] clone_execute          176
[c0000007ffdbad70] do_execute_actions     496
[c0000007ffdbaf60] ovs_execute_actions    128
[c0000007ffdbafe0] ovs_dp_process_packet  208
[c0000007ffdbb0b0] ovs_vport_receive      528
[c0000007ffdbb2c0] internal_dev_xmit
                                 total = 2368
[ff49b6d4065a3628] do_execute_actions     416
[ff49b6d4065a37c8] ovs_execute_actions     48
[ff49b6d4065a37f8] ovs_dp_process_packet  112
[ff49b6d4065a3868] clone_execute           64
[ff49b6d4065a38a8] do_execute_actions     416
[ff49b6d4065a3a48] ovs_execute_actions     48
[ff49b6d4065a3a78] ovs_dp_process_packet  112
[ff49b6d4065a3ae8] ovs_vport_receive      496
[ff49b6d4065a3cd8] netdev_frame_hook
                                 total = 1712

That's more significant than I thought, nearly 40% more stack usage for
ppc even with 3 frames having large local variables that can't be
avoided for either arch.

So, x86_64 could be quite safe with its 16kB stack for the same
workload, explaining why same overflow has not been seen there.

Thanks,
Nick