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SubjectRe: [Xen-devel] [PATCH RFC V6 0/11] Paravirtualized ticketlocks
On 03/31/2012 12:07 AM, Thomas Gleixner wrote:
> On Fri, 30 Mar 2012, H. Peter Anvin wrote:
>> What is the current status of this patchset? I haven't looked at it too
>> closely because I have been focused on 3.4 up until now...
> The real question is whether these heuristics are the correct approach
> or not.
> If I look at it from the non virtualized kernel side then this is ass
> backwards. We know already that we are holding a spinlock which might
> cause other (v)cpus going into eternal spin. The non virtualized
> kernel solves this by disabling preemption and therefor getting out of
> the critical section as fast as possible,
> The virtualization problem reminds me a lot of the problem which RT
> kernels are observing where non raw spinlocks are turned into
> "sleeping spinlocks" and therefor can cause throughput issues for non
> RT workloads.
> Though the virtualized situation is even worse. Any preempted guest
> section which holds a spinlock is prone to cause unbound delays.
> The paravirt ticketlock solution can only mitigate the problem, but
> not solve it. With massive overcommit there is always a way to trigger
> worst case scenarious unless you are educating the scheduler to cope
> with that.
> So if we need to fiddle with the scheduler and frankly that's the only
> way to get a real gain (the numbers, which are achieved by this
> patches, are not that impressive) then the question arises whether we
> should turn the whole thing around.
> I know that Peter is going to go berserk on me, but if we are running
> a paravirt guest then it's simple to provide a mechanism which allows
> the host (aka hypervisor) to check that in the guest just by looking
> at some global state.
> So if a guest exits due to an external event it's easy to inspect the
> state of that guest and avoid to schedule away when it was interrupted
> in a spinlock held section. That guest/host shared state needs to be
> modified to indicate the guest to invoke an exit when the last nested
> lock has been released.
> Of course this needs to be time bound, so a rogue guest cannot
> monopolize the cpu forever, but that's the least to worry about
> problem simply because a guest which does not get out of a spinlocked
> region within a certain amount of time is borked and elegible to
> killing anyway.
> Thoughts ?

I used this approach in 2008:

It worked very well, but it was rejected at that time. I wouldn't mind trying
it again if there is some support from your side. :-)



Juergen Gross Principal Developer Operating Systems
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