Re: [Xen-devel] [PATCH RFC V6 0/11] Paravirtualized ticketlocks

[Marcelo Tosatti <mtosatti@xxxxxxxxxx>]

On Sat, Mar 31, 2012 at 12:07:58AM +0200, 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.

Remember that the host is scheduling other processes than vcpus of guests. 

The case where a higher priority task (whatever that task is) interrupts
a vcpu which holds a spinlock should be frequent, in a overcommit
scenario. Whenever that is the case, other vcpus _must_ be able to stop
spinning. 

Now extrapolate that to guests with large number of vcpus. There is no
replacement for sleep-in-hypervisor-instead-of-spin.

> 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 ?
> 
> Thanks,
> 
>       tglx

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