Re: [PATCH 4/6] kvm tools: Add rwlock wrapper

[Avi Kivity <avi@redhat.com>]

On 05/27/2011 02:07 PM, Ingo Molnar wrote:
> * Ingo Molnar<mingo@elte.hu>  wrote:
>
> >  >  This code is very much tied with the kernel scheduler. [...]
> >
> >  It would not be particularly complex to enable user-space to
> >  request a callback on context switch events.
> >
> >  I was thinking on and off about allowing perf events to generate a
> >  per sampling event notification signal on specific events, such as
> >  page faults or context switches.
>
> I was thinking about that on and off so loudly that Peter implemented
> it long ago via fasync support on the perf event fd! :-)
>
> So if you set a notification signal via fcntl(F_SETOWN) on the
> scheduler context switch event fd, the user-space RCU code will get a
> signal on every context switch.
>

Context switches are completely uninteresting for userspace rcu:

   rcu_read_lock();
   ---> context switch

have we learned anything from that?  no.  User code is always 
preemptible and migratable.  If rcu_read_lock() prevented migration 
somehow, then we'd know that a context switch means we've started a 
grace period for this thread.  But it doesn't, so we don't.

What's needed are explicit notifications about grace periods.  For the 
vcpu threads, calling KVM_VCPU_RUN seems like a good point.  For I/O 
threads, completion of processing of an event is also a good point.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.