Re: [Xen-devel] [PATCH RFC 01/49] xen/sched: call cpu_disable_scheduler() via cpu notifier

[Julien Grall <julien.grall@xxxxxxx>]

Hi Juergen,

On 4/1/19 11:37 AM, Juergen Gross wrote:
> On 01/04/2019 12:29, Julien Grall wrote:
> > Hi,
> >
> > On 4/1/19 10:40 AM, Juergen Gross wrote:
> > > On 01/04/2019 11:21, Julien Grall wrote:
> > > > Hi,
> > > >
> > > > On 3/29/19 3:08 PM, Juergen Gross wrote:
> > > > > cpu_disable_scheduler() is being called from __cpu_disable() today.
> > > > > There is no need to execute it on the cpu just being disabled, so use
> > > > > the CPU_DEAD case of the cpu notifier chain. Moving the call out of
> > > > > stop_machine() context is fine, as we just need to hold the domain RCU
> > > > > lock and need the scheduler percpu data to be still allocated.
> > > > >
> > > > > Add another hook for CPU_DOWN_PREPARE to bail out early in case
> > > > > cpu_disable_scheduler() would fail. This will avoid crashes in rare
> > > > > cases for cpu hotplug or suspend.
> > > > >
> > > > > While at it remove a superfluous smp_mb() in the ARM __cpu_disable()
> > > > > incarnation.
> > > > This is not obvious why the smp_mb() is superfluous. Can you please
> > > > provide more details on why this is not necessary?
> > > cpumask_clear_cpu() should already have the needed semantics, no?
> > > It is based on clear_bit() which is defined to be atomic.
> > atomicity does not mean the store/load cannot be re-ordered by the CPU.
> > You would need a barrier to prevent re-ordering.
> >
> > cpumask_clear_cpu() and clear_bit() does not contain any barrier, so
> > store/load can be re-ordered.
> Uh, couldn't this lead to problems, e.g. in vcpu_block()? The comment
> there suggests the sequence of setting the blocked bit and doing the
> test is important for avoiding a race...
Hmmm... looking at the other usage (such as in do_poll), on non-x86 
platform, there is a smp_mb() between set_bit(...) and checking the 
event with a similar comment above.
I don't know enough the scheduler code to know why the barrier is 
needed. But for consistency, it seems to me the smp_mb() would be 
required in vcpu_block() as well.
Also, it is quite interesting that the barrier is not presence for x86. 
If I understand correctly the comment on top of set_bit/clear_bit, it 
could as well be re-ordered. So we seem to relying on the underlying 
implementation of set_bit/clear_bit.
Wouldn't it make sense to try to uniformize the semantics? Maybe by 
introducing a new helper?
Cheers,

--
Julien Grall

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