Re: [Qemu-devel] [RFC PATCH 13/26] cpus: only take BQL for sleeping threads

[Pavel Dovgalyuk]

> From: Paolo Bonzini [mailto:address@hidden
> > From: "David Hildenbrand" <address@hidden>
> > On 02.11.2017 12:08, Paolo Bonzini wrote:
> > > On 31/10/2017 12:26, Pavel Dovgalyuk wrote:
> > >> From: Alex Bennée <address@hidden>
> > >>
> > >> Now the only real need to hold the BQL is for when we sleep on the
> > >> cpu->halt conditional. The lock is actually dropped while the thread
> > >> sleeps so the actual window for contention is pretty small. This also
> > >> means we can remove the special case hack for exclusive work and
> > >> simply declare that work no longer has an implicit BQL held. This
> > >> isn't a major problem async work is generally only changing things in
> > >> the context of its own vCPU. If it needs to work across vCPUs it
> > >> should be using the exclusive mechanism or possibly taking the lock
> > >> itself.
> > >>
> > >> Signed-off-by: Alex Bennée <address@hidden>
> > >> Tested-by: Pavel Dovgalyuk <address@hidden>
> > >
> > > At least cpu_throttle_thread would fail with this patch.
> > >
> > > Also I am not sure if the s390 SIGP handlers are ready for this.
> > >
> >
> > We have a global lock to the SIGP "facility". However we need the BQL in
> > order to inject interrupts into CPUs (otherwise it would trigger an
> > assert when injecting).
> >
> > We inject Restart and Stop interrupts from run_on_cpu. This requires the
> > BQL. So Paolo should be right, this change would break s390x.
> 
> I had some patches to access interrupt_request with the atomic builtins.  If
> Pavel can first extract the other changes to the icount mechanism, I can
> update them.

What changes do you mean here?
I'm not sure that I understand clearly how threads interact with BQL.
These patches were authored by Alex and we'll have to get him into the 
discussion.

Pavel Dovgalyuk