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Subject[PATCH RFC] cpu: No more __stop_machine() in _cpu_down()
The _cpu_down() function invoked as part of the CPU-hotplug offlining
process currently invokes __stop_machine(), which is slow and inflicts
substantial real-time latencies on the entire system. This patch
substitutes stop_cpus() for __stop_machine() in order to improve
both performance and real-time latency.

This is currently unsafe, because there are a number of uses of
preempt_disable() that are intended to block CPU-hotplug offlining.
These will be fixed, but in the meantime, this commit is one way to help
locate them. It nevertheless passes light rcutorture/hotplug stress
testing. Meaning that we should not be relying on pure testing to find
places where people are relying on preemption disabling to block CPUs
from going offline. ;-)

Not-yet-signed-off-by: Paul E. McKenney <>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index a4eb522..47e63a0 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -243,13 +243,18 @@ static int __ref take_cpu_down(void *_param)
struct take_cpu_down_param *param = _param;
int err;
+ unsigned long flags;

/* Ensure this CPU doesn't handle any more interrupts. */
+ local_irq_save(flags);
err = __cpu_disable();
- if (err < 0)
+ if (err < 0) {
+ local_irq_restore(flags);
return err;
+ }

cpu_notify(CPU_DYING | param->mod, param->hcpu);
+ local_irq_restore(flags);
return 0;

@@ -281,7 +286,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
goto out_release;

- err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
+ err = stop_cpus(cpumask_of(cpu), take_cpu_down, &tcd_param);
if (err) {
/* CPU didn't die: tell everyone. Can't complain. */
cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);

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