netdev - RE: [PATCH net] net: mana: Fix doorbell out of order violation and avoid unnecessary doorbell rings

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Message-ID:
 <SJ0PR21MB1324B5BD9AFCE00B271F198ABFBF2@SJ0PR21MB1324.namprd21.prod.outlook.com>
Date: Tue, 6 Aug 2024 19:28:15 +0000
From: Dexuan Cui <decui@...rosoft.com>
To: Long Li <longli@...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>, Paolo Abeni <pabeni@...hat.com>, Shradha Gupta
	<shradhagupta@...ux.microsoft.com>, Simon Horman <horms@...nel.org>,
	Konstantin Taranov <kotaranov@...rosoft.com>, Souradeep Chakrabarti
	<schakrabarti@...ux.microsoft.com>, Erick Archer <erick.archer@...look.com>,
	"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>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
CC: "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH net] net: mana: Fix doorbell out of order violation and
 avoid unnecessary doorbell rings

> From: longli@...uxonhyperv.com <longli@...uxonhyperv.com>
> Sent: Monday, August 5, 2024 4:38 PM
> [...]
> After napi_complete_done() is called, another NAPI may be running on
> another CPU and ring the doorbell before the current CPU does. When

Can you please share more details about "another NAPI"? Is it about busy_poll?

> combined with unnecessary rings when there is no need to ARM the CQ, this
> triggers error paths in the hardware.
>
> Fix this by always ring the doorbell in sequence and avoid unnecessary
> rings.

I'm not sure what "error paths in the hardware" means. It's better to describe
the user-visible consequence.

Maybe this is clearer:

When there is no need to arm the CQ from NAPI's perspective, the driver must
not combine "too many" arming operations due to a MANA hardware requirement:
the driver must ring the doorbell at least once within every 8 wraparounds of the CQ,
otherwise "XXX" would happen. //Dexuan: I don't know what the "XXX" is

Add a per-CQ counter cq->work_done_since_doorbell, and make sure the CQ is
armed within 4 wraparounds of the CQ. //Dexuan: why not 8 or 7?

 
> +	if (w < cq->budget) {
> +		mana_gd_ring_cq(gdma_queue, SET_ARM_BIT);
> +		cq->work_done_since_doorbell = 0;
> +		napi_complete_done(&cq->napi, w);
> +	} else if (cq->work_done_since_doorbell >
> +		   cq->gdma_cq->queue_size / COMP_ENTRY_SIZE * 4) {
> +		/* MANA hardware requires at least one doorbell ring every 8
s/ring every 8/arming within every 8/ ?

> +		 * wraparounds of CQ even there is no need to ARM. This
> driver

s/ARM/arming/ ?
s/even/even if/ ?

Thanks,
Dexuan