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SubjectRe: [PATCH RFC V6 0/11] Paravirtualized ticketlocks
On 03/31/2012 01:07 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.

Interesting idea (I think it has been raised before btw, don't recall by

One thing about it is that it can give many false positives. Consider a
fine-grained spinlock that is being accessed by many threads. That is,
the lock is taken and released with high frequency, but there is no
contention, because each vcpu is accessing a different instance. So the
host scheduler will needlessly delay preemption of vcpus that happen to
be holding a lock, even though this gains nothing.

A second issue may happen with a lock that is taken and released with
high frequency, with a high hold percentage. The host scheduler may
always sample the guest in a held state, leading it to conclude that
it's exceeding its timeout when in fact the lock is held for a short
time only.

> 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.

Hopefully not killing! Just because a guest doesn't scale well, or even
if it's deadlocked, doesn't mean it should be killed. Just preempt it.

> Thoughts ?

It's certainly interesting. Maybe a combination is worthwhile - prevent
lockholder preemption for a short period of time AND put waiters to
sleep in case that period is insufficient to release the lock.

error compiling committee.c: too many arguments to function

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