Messages in this thread | | | Date | Wed, 23 Apr 2008 03:11:54 +0200 | From | Nick Piggin <> | Subject | Re: [PATCH 2/11] x86: convert to generic helpers for IPI function calls |
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On Tue, Apr 22, 2008 at 12:50:30PM -0700, Linus Torvalds wrote: > > > On Tue, 22 Apr 2008, Ingo Molnar wrote: > > > > ok. In which case the reschedule vector could be consolidated into that > > as well (it's just a special single-CPU call). Then there would be no > > new vector allocations needed at all, just the renaming of > > RESCHEDULE_VECTOR to something more generic. > > Yes. > > Btw, don't get me wrong - I'm not against multiple vectors per se. I just > wonder if there is any real reason for the code duplication. > > And there certainly *can* be tons of valid reasons for it. For example, > some of the LAPIC can only have something like two pending interrupts per > vector, and after that IPI's would get lost. > > However, since the queuing is actually done with the data structures, I > don't think it matters for the IPI's - they don't need any hardware > queuing at all, afaik, since even if two IPI's would be merged into one > (due to lack of hw queueing) the IPI handling code still has its list of > events, so it doesn't matter. > > And performance can be a valid reason ("too expensive to check the shared > queue if we only have per-cpu events"), although I$ issues can cause that > argument to go both ways. > > I was also wondering whether there are deadlock issues (ie one type of IPI > has to complete even if a lock is held for the other type). > > So I don't dislike the patch per se, I just wanted to understand _why_ the > IPI's wanted separate vectors.
The "too expensive to check the shared queue" is one aspect of it. The shared queue need not have events *for us* (at least, unless Jens has changed the implementation a bit) but it can still have events that we would need to check through.
I don't think deadlock is a problem (any more than with multiple vectors).
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