Re: [PATCH 1/3] IB/core: use IB_CQ_REPORT_MISSED_EVENTS to avoid missed CQ callbacks

[Roland Dreier <rdreier-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

 > The way I understand the sequence of events w/o the
 > patch is:
 > 
 > ib_req_notify_cq(IB_CQ_NEXT_COMP)
 > CQE 1 added to queue
 > 	callback scheduled via tasklet
 > 	future callbacks disarmed
 > callback function calls ib_req_notify_cq(IB_CQ_NEXT_COMP)
 > callback function calls ib_poll_cq() and gets CQE 1
 > callback function calls ib_poll_cq() and gets none
 > CQE 2 added to queue via IRQ
 > 	callback scheduled via tasklet
 > 	future callbacks disarmed
 > callback function returns
 > some time later, tasklet runs and calls CQ callback function.
 > callback function calls ib_req_notify_cq(IB_CQ_NEXT_COMP)
 > callback function calls ib_poll_cq() and gets CQE 2
 > 
 > Since a tasklet or workqueue can be scheduled in the
 > callback function, the second CQE isn't "missed" but
 > there is a scheduling delay before the callback happens
 > and sees CQE 2.

Sure but a tasklet at least is likely to have pretty low latency.  And
maybe there's a better chance of batching completion processing by
introducing a little delay there.  So maybe it helps, maybe it doesn't.

 > I'm willing to withdraw the 1-3 patches.
 > 
 > I still don't understand why the timing difference matters
 > to RDS.

I'd like to have some data showing an improvement before making this
change.  And as I already said it would be good to know what's really
going on with RDS before we change that.

 - R.
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Roland Dreier <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org> || For corporate legal information go to:
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