Re: [Xen-devel] [PATCH 0/3] VMX: Properly handle pi descriptor and per-cpu blocking list

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2016-06-23 at 12:33 +0000, Wu, Feng wrote:
> > -----Original Message-----
> > From: Dario Faggioli [mailto:dario.faggioli@xxxxxxxxxx]
> > 
> > It goes through all the vcpus of all domains, and does not check or
> > care whether they are running, runnable or blocked.
> > 
> > Let's look at this in some more details. So, let's assume that
> > processor 5 is going away, and that you have the following vcpus
> > around:
> > 
> >  d0v0 : v->processor = 5, running on cpu 5
> >  d0v1 : v->processor = 4, running on cpu 4
> >  d1v0 : v->processor = 5, runnable but not running
> >  d2v3 : v->processor = 5, blocked
> > 
> > for d0v0, we do:
> >   cpu_disable_scheduler(5)
> >     set_bit(_VPF_migrating, d0v0->pause_flags);
> >     vcpu_sleep_nosync(d0v0);
> >       SCHED_OP(sleep, d0v0);
> >         csched_vcpu_sleep(d0v0)
> >           cpu_raise_softirq(5, SCHEDULE_SOFTIRQ);
> >     vcpu_migrate(d0v0);
> >       if ( v->is_running || ...) // assume v->is_running is true
> >         return
> Hi Dario, after read this mail again, I get another question,
> could you please help me out?
> 
> In the above code flow, we return in vcpu_migrate(d0v0) because
> v->is_running == 1, after vcpu_migrate() return, we check:
> 
>     if ( v->processor == cpu )
>         ret = -EAGAIN; 
> 
> In my understand in the above case, 'v->processor' is likely equal to
> 'cpu', hence return -EAGAIN. However, in __cpu_disable(), there is
> some check as below:
> 
>     if ( cpu_disable_scheduler(cpu) )
>         BUG();
> 
Right. But, as the comment inside cpu_disable_scheduler() itself says,
we only return -EAGAIN in case we are calling cpu_disable_scheduler for
removing a pCPU from a cpupool.

In that case, we do not use __cpu_disable(), and hence we can safely
return an error value. In that case, in fact, the caller of
cpu_disable_scheduler() is cpupool_unassign_cpu_helprer(), which does
what's necessary to deal with such error.

> Might we hit the BUG() in the above case? 
>
No, because we call cpu_disable_scheduler() from __cpu_disable(), only
when system state is SYS_STATE_suspend already, and hence we take the
then branch of the 'if', which does never return an error.

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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