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

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Date:	Sat, 14 Nov 2015 08:08:49 +0100
From:	Christoph Hellwig <hch@....de>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	Christoph Hellwig <hch@....de>, 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 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.

> > +	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 understand several drivers are not using a hard irq context for the
> comp_handler call back. Is there any way to exploit that in this new
> API so we don't have to do so many context switches? Ie if the driver
> already is using a softirq when calling comp_handler can we somehow
> just rig ib_poll_handler directly and avoid the overhead? (Future)

Let's say this API makes it possible.  I still don't think moving the
whole budget and rearm logic into the LLD is necessarily a good idea
if we can avoid it.
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