Re: [PATCH bpf-next v1 0/2] xdp: recycle Page Pool backed skbs built from XDP frames

Alexander Lobakin
2023-03-03T11:34:14Z

From: Jesper Dangaard Brouer <jbrouer@redhat.com>
Date: Fri, 3 Mar 2023 11:39:06 +0100

>
> On 01/03/2023 17.03, Alexander Lobakin wrote:
>> Yeah, I still remember that "Who needs cpumap nowadays" (c), but anyway.
>>
>> __xdp_build_skb_from_frame() missed the moment when the networking stack
>> became able to recycle skb pages backed by a Page Pool. This was making
>                                                ^^^^^^^^^
> When talking about page_pool, can we write "page_pool" instead of
> capitalized "Page Pool", please. I looked through the git log, and here
> we all used "page_pool".

Ah okay, no prob :D Yeah, that's probably more correct. "Page Pool" is
the name of the API, while page_pool is an entity we create via
page_pool_create().

>
>> e.g. cpumap redirect even less effective than simple %XDP_PASS. veth was
>> also affected in some scenarios.
>
> Thanks for working on closing this gap :-)
>
>> A lot of drivers use skb_mark_for_recycle() already, it's been almost
>> two years and seems like there are no issues in using it in the generic
>> code too. {__,}xdp_release_frame() can be then removed as it losts its
>> last user.
>> Page Pool becomes then zero-alloc (or almost) in the abovementioned
>> cases, too. Other memory type models (who needs them at this point)
>> have no changes.
>>
>> Some numbers on 1 Xeon Platinum core bombed with 27 Mpps of 64-byte
>> IPv6 UDP:
>
> What NIC driver?

IAVF with XDP, the series adding XDP support will be sent in a couple
weeks, WIP can be found on my open GH[0].

>
>>
>> Plain %XDP_PASS on baseline, Page Pool driver:
>>
>> src cpu Rx     drops  dst cpu Rx
>>    2.1 Mpps       N/A    2.1 Mpps
>>
>> cpumap redirect (w/o leaving its node) on baseline:
>>
>>    6.8 Mpps  5.0 Mpps    1.8 Mpps
>>
>> cpumap redirect with skb PP recycling:
>>
>>    7.9 Mpps  5.7 Mpps    2.2 Mpps   +22%
>>
>
> It is of cause awesome, that cpumap SKBs are faster than normal SKB path.

That's the point of cpumap redirect, right? You separate NAPI poll / IRQ
handling from the skb networking stack traveling to a different CPU,
including page freeing (or recycling). That takes a lot of load from the
source CPU. 0.1 Mpps is not the highest difference I got, cpumap
redirect can boost up to 0.5 Mpps IIRC.

> I do wonder where the +22% number comes from?

(2.2 - 1.8) / 1.8 * 100%. I compare baseline cpumap redirect
before/after here :)

>
>> Alexander Lobakin (2):
>>    xdp: recycle Page Pool backed skbs built from XDP frames
>>    xdp: remove unused {__,}xdp_release_frame()
>>
>>   include/net/xdp.h | 29 -----------------------------
>>   net/core/xdp.c    | 19 ++-----------------
>>   2 files changed, 2 insertions(+), 46 deletions(-)
>>
>

There's a build failure on non-PP systems due to skb_mark_for_recycle()
being declared only when CONFIG_PAGE_POOL is set. I'll spin v2 in a bit.

[0] https://github.com/alobakin/linux/commits/iavf-xdp

Thanks,
Olek