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SubjectRe: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs
On Fri, Aug 02, 2019 at 08:15:01AM -0700, Paul E. McKenney wrote:
> The multi_cpu_stop() function relies on the scheduler to gain control from
> whatever is running on the various online CPUs, including any nohz_full
> CPUs running long loops in kernel-mode code. Lack of the scheduler-clock
> interrupt on such CPUs can delay multi_cpu_stop() for several minutes
> and can also result in RCU CPU stall warnings. This commit therefore
> causes multi_cpu_stop() to enable the scheduler-clock interrupt on all
> online CPUs.
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> ---
> kernel/stop_machine.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index b4f83f7bdf86..a2659f61ed92 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -20,6 +20,7 @@
> #include <linux/smpboot.h>
> #include <linux/atomic.h>
> #include <linux/nmi.h>
> +#include <linux/tick.h>
> #include <linux/sched/wake_q.h>
>
> /*
> @@ -187,15 +188,19 @@ static int multi_cpu_stop(void *data)
> {
> struct multi_stop_data *msdata = data;
> enum multi_stop_state curstate = MULTI_STOP_NONE;
> - int cpu = smp_processor_id(), err = 0;
> + int cpu, err = 0;
> const struct cpumask *cpumask;
> unsigned long flags;
> bool is_active;
>
> + for_each_online_cpu(cpu)
> + tick_nohz_dep_set_cpu(cpu, TICK_DEP_MASK_RCU);

Looks like it's not the right fix but, should you ever need to set an
all-CPUs (system wide) tick dependency in the future, you can use tick_set_dep().

Thanks.

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