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

[Paolo Abeni]

On Thu, 2023-06-29 at 18:18 +0000, Long Li wrote:
> > Subject: Re: [Patch v3] net: mana: Batch ringing RX queue doorbell
> > on receiving
> > packets
> > 
> > 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).
> 
> This issue is a blocker for usage on 200G physical link. I think it
> can be categorized as a fix.

Let me ask the question the other way around: is there any specific
reason to have this fix into 6.5 and all the way back to 5.13?
Especially the latest bit (CC-ing stable) looks at least debatable.

Thanks,

Paolo