linux-kernel - Re: [PATCH v2] net: mana: Batch ringing RX queue doorbell on receiving packets

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Message-ID: <20230624151037.699c50c6@kernel.org>
Date:   Sat, 24 Jun 2023 15:10:37 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     longli@...uxonhyperv.com
Cc:     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>,
        Paolo Abeni <pabeni@...hat.com>, 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 v2] net: mana: Batch ringing RX queue doorbell on
 receiving packets

On Thu, 22 Jun 2023 09:22:36 -0700 longli@...uxonhyperv.com wrote:
> It's inefficient to ring the doorbell page every time a WQE is posted to
> the received queue.
> 
> 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().
> 
> Tests showed no regression in network latency benchmarks.
> 
> Cc: stable@...r.kernel.org
> Fixes: ca9c54d2d6a5 ("net: mana: Add a driver for Microsoft Azure Network Adapter (MANA)")

If this is supposed to be a fix, you need to clearly explain what the
performance loss was, so that backporters can make an informed decision.

>  drivers/net/ethernet/microsoft/mana/gdma_main.c |  5 ++++-
>  drivers/net/ethernet/microsoft/mana/mana_en.c   | 10 ++++++++--
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/microsoft/mana/gdma_main.c b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> index 8f3f78b68592..ef11d09a3655 100644
> --- a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> +++ b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> @@ -300,8 +300,11 @@ static void mana_gd_ring_doorbell(struct gdma_context *gc, u32 db_index,
>  
>  void mana_gd_wq_ring_doorbell(struct gdma_context *gc, struct gdma_queue *queue)
>  {
> +	/* BNIC Spec specifies that client should set 0 for rq.wqe_cnt
> +	 * This value is not used in sq
> +	 */
>  	mana_gd_ring_doorbell(gc, queue->gdma_dev->doorbell, queue->type,
> -			      queue->id, queue->head * GDMA_WQE_BU_SIZE, 1);
> +			      queue->id, queue->head * GDMA_WQE_BU_SIZE, 0);
>  }

This change needs to be explained in the commit message, or should be 
a separate patch.
-- 
pw-bot: cr