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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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