On Thu, Mar 16, 2017 at 04:53:10PM +0100, Peter Zijlstra wrote:
> 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.ga11...@linux.vnet.ibm.com
>
> 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).
Very good! I am re-runnning rcutorture on -rc2 (without your patch)
and will let you know how it goes.
> 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.
Sounds good!
Thanx, Paul
