On Thu, Mar 16, 2017 at 02:00:02PM -0700, Paul E. McKenney wrote:
> On Thu, Mar 16, 2017 at 09:57:48AM -0700, Paul E. McKenney wrote:
> > 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.
>
> And -rc2 works just fine on rcutorture, thank you again!
>
> Thanx, Paul
>
> > > 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!
Ah, and I dropped this patch when rebasing to v4.11-rc2 in anticipation
of your Changelog amendments:
sched/clock: Some clear_sched_clock_stable() vs hotplug wobbles
Please let me know if you would instead prefer me to keep carrying the
current version.
Thanx, Paul
