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

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Message-ID: <36c95dd6babb2202f70594d5dde13493af62dcad.camel@redhat.com>
Date:   Thu, 29 Jun 2023 10:42:34 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     longli@...uxonhyperv.com, Jason Gunthorpe <jgg@...pe.ca>,
        Leon Romanovsky <leon@...nel.org>,
        Ajay Sharma <sharmaajay@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     linux-rdma@...r.kernel.org, linux-hyperv@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Long Li <longli@...rosoft.com>, stable@...r.kernel.org
Subject: Re: [Patch v3] net: mana: Batch ringing RX queue doorbell on
 receiving packets

On Mon, 2023-06-26 at 16:57 -0700, longli@...uxonhyperv.com wrote:
> From: Long Li <longli@...rosoft.com>
> 
> 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@...r.kernel.org
> 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