Re: [PATCH RFC 0/4] Paravirtual spinlocks

[Jeremy Fitzhardinge <jeremy@goop.org>]

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