Re: [PATCH net-next] net/mlx5e: Reuse per-RQ XDP buffer to avoid stack zeroing overhead

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 16/05/2025 15.47, Tariq Toukan wrote:
> 
> 
> On 15/05/2025 3:26, Alexei Starovoitov wrote:
>> On Wed, May 14, 2025 at 1:04 PM Tariq Toukan <tariqt@nvidia.com> wrote:
>>>
>>> From: Carolina Jubran <cjubran@nvidia.com>
>>>
>>> CONFIG_INIT_STACK_ALL_ZERO introduces a performance cost by
>>> zero-initializing all stack variables on function entry. The mlx5 XDP
>>> RX path previously allocated a struct mlx5e_xdp_buff on the stack per
>>> received CQE, resulting in measurable performance degradation under
>>> this config.
>>>
>>> This patch reuses a mlx5e_xdp_buff stored in the mlx5e_rq struct,
>>> avoiding per-CQE stack allocations and repeated zeroing.
>>>
>>> With this change, XDP_DROP and XDP_TX performance matches that of
>>> kernels built without CONFIG_INIT_STACK_ALL_ZERO.
>>>
>>> Performance was measured on a ConnectX-6Dx using a single RX channel
>>> (1 CPU at 100% usage) at ~50 Mpps. The baseline results were taken from
>>> net-next-6.15.
>>>
>>> Stack zeroing disabled:
>>> - XDP_DROP:
>>>      * baseline:                     31.47 Mpps
>>>      * baseline + per-RQ allocation: 32.31 Mpps (+2.68%)
>>>

31.47 Mpps = 31.77 nanosec per packet
32.31 Mpps = 30.95 nanosec per packet
Improvement:  0.82 nanosec faster

>>> - XDP_TX:
>>>      * baseline:                     12.41 Mpps
>>>      * baseline + per-RQ allocation: 12.95 Mpps (+4.30%)
>>

The XDP_TX number are actually lower than I expected.
Hmm... I wonder if we regressed here(?)

12.41 Mpps = 80.58 nanosec per packet
12.95 Mpps = 77.22 nanosec per packet
Improvement:  3.36 nanosec faster

>> Looks good, but where are these gains coming from ?
>> The patch just moves mxbuf from stack to rq.
>> The number of operations should really be the same.
>>
> 
> I guess it's cache related. Hot/cold areas, alignments, movement of 
> other fields in the mlx5e_rq structure...

The improvements for XDP_DROP (see calc above) in nanosec is so small
that it is hard to measure accurately/stable on any system.

The improvement for XDP_TX is above 2 nanosec, which looks like an 
actual improvement...


>>> Stack zeroing enabled:
>>> - XDP_DROP:
>>>      * baseline:                     24.32 Mpps
>>>      * baseline + per-RQ allocation: 32.27 Mpps (+32.7%)
>>
>> This part makes sense.
> 

Yes, this makes sense as it is a measurable improvement.

24.32 Mpps = 41.12 nanosec per packet
32.27 Mpps = 30.99 nanosec per packet
Improvement: 10.13 nanosec faster

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>

--Jesper