Messages in this thread | | | Date | Wed, 04 Jan 2012 23:41:29 +0200 | From | Avi Kivity <> | Subject | Re: [RFC PATCH 0/4] Gang scheduling in CFS |
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On 01/04/2012 11:31 PM, Peter Zijlstra wrote: > On Wed, 2012-01-04 at 19:16 +0200, Avi Kivity wrote: > > > > > > I think we can solve it at the guest level. The paravirt ticketlock > > stuff introduces wait/wake calls (actually wait is just a HLT > > instruction); we could spin for a while, then HLT until the other side > > wakes us. We should do this for all sites that busy wait. > > > This is all TLB invalidates, right? > > So why wait for non-running vcpus at all? That is, why not paravirt the > TLB flush such that the invalidate marks the non-running VCPU's state so > that on resume it will first flush its TLBs. That way you don't have to > wake it up and wait for it to invalidate its TLBs.
That's what Xen does, but it's tricky. For example get_user_pages_fast() depends on the IPI to hold off page freeing, if we paravirt it we have to take that into consideration.
> Or am I like totally missing the point (I am after all reading the > thread backwards and I haven't yet fully paged the kernel stuff back > into my brain).
You aren't, and I bet those kernel pages are unswappable anyway.
> I guess tagging remote VCPU state like that might be somewhat tricky.. > but it seems worth considering, the whole wake and wait for flush thing > seems daft.
It's nasty, but then so is paravirt. It's hard to get right, and it has a tendency to cause performance regressions as hardware improves.
-- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
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