Re: Null scheduler and vwfi native problem

[Julien Grall <julien@xxxxxxx>]

Hi Dario,

On 21/01/2021 23:35, Dario Faggioli wrote:
> On Thu, 2021-01-21 at 19:40 +0000, Julien Grall wrote:
> > Hi Dario,
> Hi!
>
> > On 21/01/2021 18:32, Dario Faggioli wrote:
> > > On Thu, 2021-01-21 at 11:54 +0100, Anders Törnqvist wrote:
> > > >    
> > > > https://lists.xenproject.org/archives/html/xen-devel/2018-09/msg01213.html
> > > > .
> > > Right. Back then, PCI passthrough was involved, if I remember
> > > correctly. Is it the case for you as well?
> > PCI passthrough is not yet supported on Arm :). However, the bug was
> > reported with platform device passthrough.
> Yeah, well... That! Which indeed is not PCI. Sorry for the terminology
> mismatch. :-)
>
> > > Well, I'll think about it. >
> > > > Starting the system without "sched=null vwfi=native" does not
> > > > result
> > > > in
> > > > the problem.
> > > Ok, how about, if you're up for some more testing:
> > >
> > >    - booting with "sched=null" but not with "vwfi=native"
> > >    - booting with "sched=null vwfi=native" but not doing the IRQ
> > >      passthrough that you mentioned above
> > >
> > > ?
> > I think we can skip the testing as the bug was fully diagnostics back
> > then. Unfortunately, I don't think a patch was ever posted.
> True. But an hackish debug patch was provided and, back then, it
> worked.
>
> OTOH, Anders seems to be reporting that such a patch did not work here.
> I also continue to think that we're facing the same or a very similar
> problem... But I'm curious why applying the patch did not help this
> time. And that's why I asked for more testing.
I wonder if this is because your patch doesn't modify rsinterval. So 
even if we call force_quiescent_state(), the softirq would only be 
raised for the current CPU.
I guess the following HACK could confirm the theory:

diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
index a5a27af3def0..50020bc34ddf 100644
--- a/xen/common/rcupdate.c
+++ b/xen/common/rcupdate.c
@@ -250,7 +250,7 @@ static void force_quiescent_state(struct rcu_data *rdp,
 {
     cpumask_t cpumask;
     raise_softirq(RCU_SOFTIRQ);
-    if (unlikely(rdp->qlen - rdp->last_rs_qlen > rsinterval)) {
+    if (1 || unlikely(rdp->qlen - rdp->last_rs_qlen > rsinterval)) {
         rdp->last_rs_qlen = rdp->qlen;
         /*
          * Don't send IPI to itself. With irqs disabled,

Cheers,

--
Julien Grall