Messages in this thread | | | Date | Wed, 30 Jun 2010 12:50:40 +0200 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks - base implementation |
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On 06/30/2010 11:11 AM, Peter Zijlstra wrote: >>> Uhm, I'd much rather see a single alternative implementation, not a >>> per-hypervisor lock implementation. >>> >> How would you imaging this to work? I can't see how the mechanism >> could be hypervisor agnostic. Just look at the Xen implementation >> (patch 2) - do you really see room for meaningful abstraction there? >> > I tried not to, it made my eyes bleed.. > > But from what I hear all virt people are suffering from spinlocks (and > fair spinlocks in particular), so I was thinking it'd be a good idea to > get all interested parties to collaborate on one. Fragmentation like > this hardly ever works out well. >
Yes. Now that I've looked at it a bit more closely I think these patches put way too much logic into the per-hypervisor part of the code.
> Ah, right, after looking a bit more at patch 2 I see you indeed > implement a ticket like lock. Although why you need both a ticket and a > FIFO list is beyond me. >
That appears to be a mechanism to allow it to take interrupts while spinning on the lock, which is something that stock ticket locks don't allow. If that's a useful thing to do, it should happen in the generic ticketlock code rather than in the per-hypervisor backend (otherwise we end up with all kinds of subtle differences in lock behaviour depending on the exact environment, which is just going to be messy). Even if interrupts-while-spinning isn't useful on native hardware, it is going to be equally applicable to all virtual environments.
J
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