Re: [Patch v3] net: mana: Batch ringing RX queue doorbell on receiving packets

[Paolo Abeni]

On Mon, 2023-06-26 at 16:57 -0700, longli@xxxxxxxxxxxxxxxxx wrote:
> From: Long Li <longli@xxxxxxxxxxxxx>
> 
> It's inefficient to ring the doorbell page every time a WQE is posted to
> the received queue. Excessive MMIO writes result in CPU spending more
> time waiting on LOCK instructions (atomic operations), resulting in
> poor scaling performance.
> 
> Move the code for ringing doorbell page to where after we have posted all
> WQEs to the receive queue during a callback from napi_poll().
> 
> With this change, tests showed an improvement from 120G/s to 160G/s on a
> 200G physical link, with 16 or 32 hardware queues.
> 
> Tests showed no regression in network latency benchmarks on single
> connection.
> 
> While we are making changes in this code path, change the code for
> ringing doorbell to set the WQE_COUNT to 0 for Receive Queue. The
> hardware specification specifies that it should set to 0. Although
> currently the hardware doesn't enforce the check, in the future releases
> it may do.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: ca9c54d2d6a5 ("net: mana: Add a driver for Microsoft Azure Network Adapter (MANA)")

Uhmmm... this looks like a performance improvement to me, more suitable
for the net-next tree ?!? (Note that net-next is closed now).

In any case you must avoid empty lines in the tag area.

If you really intend targeting the -net tree, please repost fixing the
above and explicitly specifying the target tree in the subj prefix.

thanks!

Paolo