Re: [PATCH] clock: Fix smp_processor_id() in preemptible bug

[Paul E. McKenney]

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" <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

This passes initial testing.  I will hammer it harder overnight, but
in the meantime:

Tested-by: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

> ---
>  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)
>