netdev - Re: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt support to mana ib driver.

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Date: Fri, 28 Jul 2023 15:02:33 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Long Li <longli@...rosoft.com>
Cc: Wei Hu <weh@...rosoft.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	Ajay Sharma <sharmaajay@...rosoft.com>,
	"leon@...nel.org" <leon@...nel.org>,
	KY Srinivasan <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>,
	Dexuan Cui <decui@...rosoft.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	"ssengar@...ux.microsoft.com" <ssengar@...ux.microsoft.com>,
	"shradhagupta@...ux.microsoft.com" <shradhagupta@...ux.microsoft.com>
Subject: Re: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt support to mana ib
 driver.

On Fri, Jul 28, 2023 at 05:51:46PM +0000, Long Li wrote:
> > Subject: Re: [PATCH v4 1/1] RDMA/mana_ib: Add EQ interrupt support to mana ib
> > driver.
> > 
> > On Fri, Jul 28, 2023 at 05:07:49PM +0000, Wei Hu wrote:
> > > Add EQ interrupt support for mana ib driver. Allocate EQs per ucontext
> > > to receive interrupt. Attach EQ when CQ is created. Call CQ interrupt
> > > handler when completion interrupt happens. EQs are destroyed when
> > > ucontext is deallocated.
> > 
> > It seems strange that interrupts would be somehow linked to a ucontext?
> > interrupts are highly limited, you can DOS the entire system if someone abuses
> > this.
> > 
> > Generally I expect a properly functioning driver to use one interrupt per CPU core.
> 
> Yes, MANA uses one interrupt per CPU. One interrupt is shared among multiple
> EQs.

So you have another multiplexing layer between the interrupt and the
EQ? That is alot of multiplexing layers..

> > You should tie the CQ to a shared EQ belong to the core that the CQ wants to have
> > affinity to.
> 
> The reason for using a separate EQ for a ucontext, is for preventing DOS. If we use
> a shared EQ, a single ucontext can storm this shared EQ affecting
> other users.

With a proper design it should not be possible. The CQ adds an entry
to the EQ and that should be rate limited by the ability of userspace
to schedule to re-arm the CQ.

Jason