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)
>
