On Mon, Mar 13, 2017 at 08:55:21AM -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. > > > > Reported-by: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> > > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > > This passes initial testing. I will hammer it harder overnight, but > in the meantime:
And it did just fine overnight, thank you! > Tested-by: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> I am guessing that you will be pushing this one for -rc3, but please let me know. Thanx, Paul > > --- > > 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) > >