Re: [Xen-devel] [PATCH v2 10/10] libxl: event: Move poller pipe emptying to the end of afterpoll

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

George Dunlap writes ("Re: [PATCH v2 10/10] libxl: event: Move poller pipe 
emptying to the end of afterpoll"):
> On 1/13/20 5:08 PM, Ian Jackson wrote:
> > If a timer event callback causes this poller to be woken (not very
> > unlikely) we would go round the poll loop twice rather than once.
> > 
> > Do the poller pipe emptying at the end; this is slightly more
> > efficient because it can't cause any callbacks, so it happens after
> > all the callbacks have been run.
> > 
> > (This pipe-emptying has to happen in afterpoll rather than the
> > apparently more logical beforepoll, because the application calling
> > beforepoll doesn't constitute a promise to actually do anything.)
> > 
> > Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> 
> I can't quite figure out: why would you end up going around the loop
> twice, and how does this fix it?

I now think this is not true and the situation I describe cannot
happen.

What I was thinking was that pollers_note_osevent_added might be
called by something from time_occurs, and that would write a byte into
the poller pipe.  But pollers_note_osevent_added doesn't wake up
pollers; it just tags them osevents_added.

I now think the spurious wakeup cannot happen because:

For this patch to make any difference, the poller pipe would have to
be woken up by something in the time scan loop in afterpoll_internal.

But poller pipes are only woken up by ao completion or by
cleanup_1_baton.

cleanup_1_baton is not called anywhere there (as an argument against:
any such call would violate the rule that cleanup_1_baton may not be
called with a poller in hand).

And as for ao completion, we would indeed wake up the poller.  But we
also mark the ao as complete, so ao_inprogress would spot
!ao_work_outstanding, and not reenter eventloop_iteration at all.
The woken-up poller would be put by ao_inprogress.

This leads me to this observation: poller_get might give you a
woken-up poller.  This is not incorrect, but it is pointless.  So
maybe I should write a patch that puts a call to
libxl__self_pipe_eatall in libxl__poller_get.

TBH I still think this patch tidies the code up a bit.

Ian.

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