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). 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.
