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? Thanx, Paul > Reported-by: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > --- > kernel/sched/clock.c | 17 ++++++++++++----- > 1 file changed, 12 insertions(+), 5 deletions(-) > > diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c > index a08795e21628..fec0f58c8dee 100644 > --- a/kernel/sched/clock.c > +++ b/kernel/sched/clock.c > @@ -141,7 +141,14 @@ static void __set_sched_clock_stable(void) > tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE); > } > > -static void __clear_sched_clock_stable(struct work_struct *work) > +static void __sched_clock_work(struct work_struct *work) > +{ > + static_branch_disable(&__sched_clock_stable); > +} > + > +static DECLARE_WORK(sched_clock_work, __sched_clock_work); > + > +static void __clear_sched_clock_stable(void) > { > struct sched_clock_data *scd = this_scd(); > > @@ -160,11 +167,11 @@ static void __clear_sched_clock_stable(struct > work_struct *work) > scd->tick_gtod, gtod_offset, > scd->tick_raw, raw_offset); > > - static_branch_disable(&__sched_clock_stable); > tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE); > -} > > -static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable); > + if (sched_clock_stable()) > + schedule_work(&sched_clock_work); > +} > > void clear_sched_clock_stable(void) > { > @@ -173,7 +180,7 @@ void clear_sched_clock_stable(void) > smp_mb(); /* matches sched_clock_init_late() */ > > if (sched_clock_running == 2) > - schedule_work(&sched_clock_work); > + __clear_sched_clock_stable(); > } > > void sched_clock_init_late(void) >