Re: [PATCH iwl-next] idpf: add flow-based XDP fallback for FWs without Tx FIFO support

[YiFei Zhu <zhuyifei@google.com>]

> From the first days of XDP implementation in idpf, it relied and
> worked solely on top of the queue-based scheduling Tx mode, which
> basically means simple FIFO. However, turned out not every firmware
> supports this mode and XDP doesn't work there at all.
> 
> Since the flow-based scheduling Tx mode is mandatory and supported
> by every FW, introduce a simple fallback guarded by a static key
> to not hurt the more performant mode. The FB mode generates a
> completion for each Tx descriptor and never guarantees that there
> won't be any out-of-order completions. Serialize that using a
> bitmap of completed descriptors and report contiguous blocks of
> free bits to match XDP and XSk expectations and avoid further
> code complication.
> 
> The usage of a bitmap on hotpath might sound scary, but this
> fallback is able to reach around 70% of the QB mode's performance,
> which is comparable to what ice gives us. The main bottlenecks are
> unlikely()s and one completion per each descriptor, while in the QB
> mode we have one completion per batch (which might contain 64 or
> even 128 frames), plus the size of the completion descriptor is
> 8 bytes in this mode (4 bytes in the QB mode), which means a lot
> of additional PCI traffic.
> 
> bloat-o-meter shows .text increase in about 2 Kb without adding new
> functions or uninlining any of the existing ones. I played a bunch
> with inlining and uninlining certain pieces or the whole fallback,
> but the compiler collapses and optimizes libeth templates so hardly
> so that each additional external call only makes things worse.
> 
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>

Tested-by: YiFei Zhu <zhuyifei@google.com>