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SubjectRe: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin with irqs disable
Peter Zijlstra wrote:
> On Mon, 2008-06-23 at 13:45 -0700, Christoph Lameter wrote:
>
>> On Mon, 23 Jun 2008, Peter Zijlstra wrote:
>>
>>
>>>> It is good that the locks are build with _trylock and _can_lock because
>>>> then we can reenable interrupts while spinning.
>>>>
>>> Well, good and bad, the turn side is that fairness schemes like ticket
>>> locks are utterly defeated.
>>>
>> True. But maybe we can make these fairness schemes more generic so that
>> they can go into core code?
>>
>
> The trouble with ticket locks is that they can't handle waiters going
> away - or in this case getting preempted by irq handlers. The one who
> took the ticket must pass it on, so if you're preempted it just sits
> there being idle, until you get back to deal with the lock.
>
> But yeah, perhaps another fairness scheme might work in the generic
> code..

Thomas Friebel presented results at the Xen Summit this week showing
that ticket locks are an absolute disaster for scalability in a virtual
environment, for a similar reason. It's a bit irritating if the lock
holder vcpu gets preempted by the hypervisor, but its much worse when
they release the lock: unless the vcpu scheduler gives a cpu to the vcpu
with the next ticket, it can waste up to N timeslices spinning.

I'm experimenting with adding pvops hook to allow you to put in new
spinlock implementations on the fly. If nothing else, it will be useful
for experimenting with different algorithms. But it definitely seems
like the old unfair lock algorithm played much better with a virtual
environment, because the next cpu to get the lock is the next one the
scheduler gives time, rather than dictating an order - and the scheduler
should mitigate the unfairness that ticket locks were designed to solve.

J


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