linux-kernel - Re: [PATCH 2/9] IB: add a proper completion queue abstraction

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Message-ID: <20151123200136.GA5640@obsidianresearch.com>
Date:	Mon, 23 Nov 2015 13:01:36 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Christoph Hellwig <hch@....de>
Cc:	linux-rdma@...r.kernel.org, sagig@....mellanox.co.il,
	bart.vanassche@...disk.com, axboe@...com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] IB: add a proper completion queue abstraction

On Sat, Nov 14, 2015 at 08:08:49AM +0100, Christoph Hellwig wrote:
> On Fri, Nov 13, 2015 at 11:25:13AM -0700, Jason Gunthorpe wrote:
> > For instance, like this, not fulling draining the cq and then doing:
> > 
> > > +	completed = __ib_process_cq(cq, budget);
> > > +	if (completed < budget) {
> > > +		irq_poll_complete(&cq->iop);
> > > +		if (ib_req_notify_cq(cq, IB_POLL_FLAGS) > 0) {
> > 
> > Doesn't seem entirely right? There is no point in calling
> > ib_req_notify_cq if the code knows there is still stuff in the CQ and
> > has already, independently, arranged for ib_poll_hander to be
> > guarenteed called.
> 
> The code only calls ib_req_notify_cq if it knowns we finished earlier than
> our budget.

Okay, having now read the whole thing, I think I see the flow now. I don't
see any holes in the above, other than it is doing a bit more work
than it needs in some edges cases because it doesn't know if the CQ is
actually empty or not.

> > > +	completed = __ib_process_cq(cq, IB_POLL_BUDGET_WORKQUEUE);
> > > +	if (completed >= IB_POLL_BUDGET_WORKQUEUE ||
> > > +	    ib_req_notify_cq(cq, IB_POLL_FLAGS) > 0)
> > > +		queue_work(ib_comp_wq, &cq->work);
> > 
> > Same comment here..
> 
> Same here - we only requeue the work item if either we processed all of
> our budget, or ib_req_notify_cq with IB_CQ_REPORT_MISSED_EVENTS told
> us that we need to poll again.

I find the if construction hard to read, but yes, it looks OK.

Jason
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