Re: [Xen-devel] [PATCH] evtchn_do_upcall: search a snapshot of level 2 bits for pending upcalls

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 29/01/2010 06:02, "Kaushik Kumar Ram" <kaushik@xxxxxxxx> wrote:

> Search a snapshot of level 2 bits for pending upcalls.
> 
> Using only a snapshot of level 1 bits can lead to unfairness
> in some cases. Consider the case where two upcalls are marked
> pending after a snapshot of level 1 bits is taken. One of these
> two upcalls can still be processed if the corresponding level 1
> bit was already set.
> 
> This is not a perfect solution since its still possible for a level 2
> bit to be set before a level 2 snapshot is taken (unless both
> snapshots are taken atomically).

When I look at this I see 512 extra bytes on the stack, and a possibly
theoretical fairness improvement. Is the improvement measurable? Even if it
is, I wonder how much of the unfairness comes from only clearing bit l1i in
l1 if active_evtchns(l1i) is zero? I understand something like that is
needed to deal with when we start scanning from halfway through an l2, but
clearly the potential impact of that check-before-the-clear is wider
ranging. If you've measured a fairness/performance win, you might get all or
most of it by making the check-before-clear more sophisticated, and at no
extra cost in stack space.

 -- Keir



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