Re: [PATCH] clock: Fix smp_processor_id() in preemptible bug

[Peter Zijlstra]

On Wed, Mar 15, 2017 at 03:58:17PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 13, 2017 at 01:46:21PM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 09, 2017 at 10:37:32AM -0800, Paul E. McKenney wrote:
> > > And it does pass light testing.  I will hammer it harder this evening.
> > > 
> > > So please send a formal patch!
> > 
> > Changed it a bit...
> > 
> > ---
> > Subject: sched/clock: Some clear_sched_clock_stable() vs hotplug wobbles
> > 
> > Paul reported two independent problems with clear_sched_clock_stable().
> > 
> >  - if we tickle it during hotplug (even though the sched_clock was
> >    already marked unstable) we'll attempt to schedule_work() and
> >    this explodes because RCU isn't watching the new CPU quite yet.
> > 
> >  - since we run all of __clear_sched_clock_stable() from workqueue
> >    context, there's a preempt problem.
> > 
> > Cure both by only doing the static_branch_disable() from a workqueue,
> > and only when it's still stable.
> > 
> > This leaves the problem what to do about hotplug actually wrecking TSC
> > though, because if it was stable and now isn't, then we will want to run
> > that work, which then will prod RCU the wrong way.  Bloody hotplug.
> 
> Would it help to do the same trick tglx applied to the hot-unplug path,
> that is IPIing some other CPU to schedule the workqueue?
> 

So I've been looking again; and I don't think its a problem anymore.

The problem you reported here:

 https://lkml.kernel.org/r/20170308221656.GA11949@xxxxxxxxxxxxxxxxxx

Should not happen after commit:

  f94c8d116997 ("sched/clock, x86/tsc: Rework the x86 'unstable' sched_clock() interface")

which landed around 4.11-rc2; so _after_ your kernel (which reported
itself as -rc1).

Because since that commit we'll never call clear_sched_clock_stable() if
tsc_unstable is set.

So I'll have to amend the Changelog somewhat.