linux-kernel - Re: [EXTERNAL] Re: [PATCH V2,net-next, 1/2] net: mana: Add support for coalesced RX packets on CQE

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Message-ID: <20260115181434.4494fe9f@kernel.org>
Date: Thu, 15 Jan 2026 18:14:34 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Haiyang Zhang <haiyangz@...ux.microsoft.com>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>, KY Srinivasan
 <kys@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui
 <DECUI@...rosoft.com>, Long Li <longli@...rosoft.com>, Andrew Lunn
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Konstantin
 Taranov <kotaranov@...rosoft.com>, Simon Horman <horms@...nel.org>, Erni
 Sri Satya Vennela <ernis@...ux.microsoft.com>, Shradha Gupta
 <shradhagupta@...ux.microsoft.com>, Saurabh Sengar
 <ssengar@...ux.microsoft.com>, Aditya Garg
 <gargaditya@...ux.microsoft.com>, Dipayaan Roy
 <dipayanroy@...ux.microsoft.com>, Shiraz Saleem
 <shirazsaleem@...rosoft.com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "linux-rdma@...r.kernel.org"
 <linux-rdma@...r.kernel.org>, Paul Rosswurm <paulros@...rosoft.com>
Subject: Re: [EXTERNAL] Re: [PATCH V2,net-next, 1/2] net: mana: Add support
 for coalesced RX packets on CQE

On Thu, 15 Jan 2026 19:57:44 +0000 Haiyang Zhang wrote:
> > > When coalescing is enabled, the device waits for packets which can
> > > have the CQE coalesced with previous packet(s). That coalescing process
> > > is finished (and a CQE written to the appropriate CQ) when the CQE is
> > > filled with 4 pkts, or time expired, or other device specific logic is
> > > satisfied.  
> > 
> > See, what I'm afraid is happening here is that you are enabling
> > completion coalescing (how long the device keeps the CQE pending).
> > Which is _not_ what rx_max_coalesced_frames controls for most NICs.
> > For most NICs rx_max_coalesced_frames controls IRQ generation logic.
> > 
> > The NIC first buffers up CQEs for typically single digit usecs, and
> > then once CQE timer exipred and writeback happened it starts an IRQ
> > coalescing timer. Once the IRQ coalescing timer expires IRQ is
> > triggered, which schedules NAPI. (broad strokes, obviously many
> > differences and optimizations exist)
> > 
> > Is my guess correct? Are you controlling CQE coalescing>
> > 
> > Can you control the timeout instead of the frame count?  
> 
> Our NIC's timeout value cannot be controlled by driver. Also, the
> timeout may be changed in future NIC HW.
> 
> So, I use the ethtool/rx-frames, which is either 1 or 4 on our
> NIC, to switch the CQE coalescing feature on/off.

I feel like this is not the first time I'm having a conversation with
you where you are not answering my direct questions, not just one
sliver. IDK why you're doing this, but being able to participate 
in  an email exchange is a bare minimum for participating upstream.
Please consider this a warning.

If I interpret your reply correctly you are indeed coalescing writeback.
You need to add a new param to the uAPI. Please add both size and
timeout. Expose the timeout as read only if your device doesn't support
controlling it per queue.