netdev - RE: [Patch v3] net: mana: Batch ringing RX queue doorbell on receiving packets

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Date:   Thu, 29 Jun 2023 18:18:37 +0000
From:   Long Li <longli@...rosoft.com>
To:     Paolo Abeni <pabeni@...hat.com>,
        "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Leon Romanovsky <leon@...nel.org>,
        Ajay Sharma <sharmaajay@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        KY 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-rdma@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [Patch v3] net: mana: Batch ringing RX queue doorbell on
 receiving packets

> 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).

This issue is a blocker for usage on 200G physical link. I think it can be categorized as a fix.

> 
> 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.

Will do, thank you.

Long

> 
> thanks!
> 
> Paolo