Rusty Russell wrote:
> On Tuesday 08 July 2008 05:07:49 Jeremy Fitzhardinge wrote:
>
>> At the most recent Xen Summit, Thomas Friebel presented a paper
>> ("Preventing Guests from Spinning Around",
>> http://xen.org/files/xensummitboston08/LHP.pdf) investigating the
>> interactions between spinlocks and virtual machines. Specifically, he
>> looked at what happens when a lock-holding VCPU gets involuntarily
>> preempted.
>>
>
> I find it interesting that gang scheduling the guest was not suggested as an
> obvious solution.
>
It's an obvious answer, but not an obvious solution. You trade off
wasting time spinning vs wasting time waiting for N vcpus to be free for
scheduling. Or something; seems much more complex, particularly if you
can do a small guest tweak to solve the problem.
> Anyway, concept looks fine; lguest's solution is more elegant of course :)
>
You could remove all mutable state and call it "erlang".
> A little disappointing that you can't patch your version inline.
Spinlock code isn't inlined currently, so I hadn't considered it. The
fast path code for both lock and unlock is nearly small enough to
consider it, but it seems a bit fiddly.
If the "spin_lock" and "spin_unlock" functions were inlined functions
which called the out of line __raw_spin_lock/unlock functions, then
after patching they would result in a direct call to the backend lock
functions, which would be exactly equivalent to what happens now (since
I hook __raw_spin_lock into calls via pv_lock_ops).
J
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