From: Daniel Xu <redacted>
Date: Thu, 08 Aug 2024 16:52:51 -0400

Hi,

On Thu, Aug 8, 2024, at 7:57 AM, Alexander Lobakin wrote:

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From: Lorenzo Bianconi <redacted>
Date: Thu, 8 Aug 2024 06:54:06 +0200

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Hi Alexander,

On Tue, Jun 28, 2022, at 12:47 PM, Alexander Lobakin wrote:

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cpumap has its own BH context based on kthread. It has a sane batch
size of 8 frames per one cycle.
GRO can be used on its own, adjust cpumap calls to the
upper stack to use GRO API instead of netif_receive_skb_list() which
processes skbs by batches, but doesn't involve GRO layer at all.
It is most beneficial when a NIC which frame come from is XDP
generic metadata-enabled, but in plenty of tests GRO performs better
than listed receiving even given that it has to calculate full frame
checksums on CPU.
As GRO passes the skbs to the upper stack in the batches of
@gro_normal_batch, i.e. 8 by default, and @skb->dev point to the
device where the frame comes from, it is enough to disable GRO
netdev feature on it to completely restore the original behaviour:
untouched frames will be being bulked and passed to the upper stack
by 8, as it was with netif_receive_skb_list().

Signed-off-by: Alexander Lobakin <redacted>
---
 kernel/bpf/cpumap.c | 43 ++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 38 insertions(+), 5 deletions(-)

AFAICT the cpumap + GRO is a good standalone improvement. I think
cpumap is still missing this.

The only concern for having GRO in cpumap without metadata from the NIC
descriptor was that when the checksum status is missing, GRO calculates
the checksum on CPU, which is not really fast.
But I remember sometimes GRO was faster despite that.

Good to know, thanks. IIUC some kind of XDP hint support landed already?

My use case could also use HW RSS hash to avoid a rehash in XDP prog.

Unfortunately, for now it's impossible to get HW metadata such as RSS
hash and checksum status in cpumap. They're implemented via kfuncs
specific to a particular netdevice and this info is available only when
running XDP prog.

But I think one solution could be:

1. We create some generic structure for cpumap, like

struct cpumap_meta {
	u32 magic;
	u32 hash;
}

2. We add such check in the cpumap code

	if (xdpf->metalen == sizeof(struct cpumap_meta) &&
	    <here we check magic>)
		skb->hash = meta->hash;

3. In XDP prog, you call Rx hints kfuncs when they're available, obtain
RSS hash and then put it in the struct cpumap_meta as XDP frame metadata.

And HW RX timestamp to not break SO_TIMESTAMPING. These two
are on one of my TODO lists. But I can’t get to them for at least
a few weeks. So free to take it if you’d like.

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I have a production use case for this now. We want to do some intelligent
RX steering and I think GRO would help over list-ified receive in some cases.
We would prefer steer in HW (and thus get existing GRO support) but not all
our NICs support it. So we need a software fallback.

Are you still interested in merging the cpumap + GRO patches?

For sure I can revive this part. I was planning to get back to this
branch and pick patches which were not related to XDP hints and send
them separately.

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Hi Daniel and Alex,

Recently I worked on a PoC to add GRO support to cpumap codebase:
- https://github.com/LorenzoBianconi/bpf-next/commit/a4b8264d5000ecf016da5a2dd9ac302deaf38b3e
  Here I added GRO support to cpumap through gro-cells.
- https://github.com/LorenzoBianconi/bpf-next/commit/da6cb32a4674aa72401c7414c9a8a0775ef41a55
  Here I added GRO support to cpumap trough napi-threaded APIs (with a some
  changes to them).

Hmm, when I was testing it, adding a whole NAPI to cpumap was sorta
overkill, that's why I separated GRO structure from &napi_struct.

Let me maybe find some free time, I would then test all 3 solutions
(mine, gro_cells, threaded NAPI) and pick/send the best?

Sounds good. Would be good to compare results.

[…]

Thanks,
Daniel

Thanks,
Olek