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

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 ?



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