netdev - Re: [PATCH v4 01/13] Linux RDMA Core Changes

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Message-ID: <20070103151823.GR6019@mellanox.co.il>
Date:	Wed, 3 Jan 2007 17:18:23 +0200
From:	"Michael S. Tsirkin" <mst@...lanox.co.il>
To:	Steve Wise <swise@...ngridcomputing.com>
Cc:	netdev@...r.kernel.org, Roland Dreier <rdreier@...co.com>,
	linux-kernel@...r.kernel.org, openib-general@...nib.org
Subject: Re: [PATCH  v4 01/13] Linux RDMA Core Changes

> > > > No, it won't need 2 transitions - just an extra function call,
> > > > so it won't hurt performance - it would improve performance.
> > > > 
> > > > ib_uverbs_req_notify_cq would call
> > > > 
> > > > 	ib_uverbs_req_notify_cq()
> > > > 	{
> > > > 			ib_set_cq_udata(cq, udata)
> > > > 			ib_req_notify_cq(cq, cmd.solicited_only ?
> > > > 				IB_CQ_SOLICITED : IB_CQ_NEXT_COMP);
> > > > 	}
> > > > 
> > > 
> > > ib_set_cq_udata() would transition into the kernel to pass in the
> > > consumer's index.  In addition, ib_req_notify_cq would also transition
> > > into the kernel since its not a bypass function for chelsio.
> > 
> > We misunderstand each other.
> > 
> > ib_uverbs_req_notify_cq is in drivers/infiniband/core/uverbs_cmd.c -
> > all this code runs inside the IB_USER_VERBS_CMD_REQ_NOTIFY_CQ command,
> > so there is a single user to kernel transition.
> > 
> 
> Oh I see. 
> 
> This seems like a lot of extra code to avoid passing one extra arg to
> the driver's req_notify_cq verb.  I'd appreciate other folk's input on
> how important they think this is.  
>
> If you insist, then I'll run some tests specifically in kernel mode and
> see how this affects mthca's req_notify performance.

This might be an interesting datapoint.

-- 
MST
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