Messages in this thread | | | Date | Wed, 30 Jun 2010 14:53:22 +0200 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks - base implementation |
| |
On 06/30/2010 01:52 PM, Jan Beulich wrote: > I fail to see that: Depending on the hypervisor's capabilities, the > two main functions could be much smaller (potentially there wouldn't > even be a need for the unlock hook in some cases),
What mechanism are you envisaging in that case?
>> 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. >> > While we do interrupt re-enabling in our pv kernels, I intentionally > didn't do this here - it complicates the code quite a bit further, and > that did seem right for an initial submission. >
Ah, I was confused by this: > + /* > + * If we interrupted another spinlock while it was blocking, make > + * sure it doesn't block (again) without re-checking the lock. > + */ > + if (spinning.prev) > + sync_set_bit(percpu_read(poll_evtchn), > + xen_shared_info->evtchn_pending); > + > +
> The list really juts is needed to not pointlessly tickle CPUs that > won't own the just released lock next anyway (or would own > it, but meanwhile went for another one where they also decided > to go into polling mode).
Did you measure that it was a particularly common case which was worth optimising for?
J
| |