Re: [PATCH RFC 0/4] Paravirtual spinlocks

Rusty Russell wrote:

On Tuesday 08 July 2008 05:07:49 Jeremy Fitzhardinge wrote:
  

quoted

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