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![Christian Borntraeger](/assets/img/avatar_default.png)
On kvm I have seen some rare hangs in stop_machine when I used more guest cpus than hosts cpus. e.g. 32 guest cpus on 1 host cpu triggered the hang quite often. I could also reproduce the problem on a 4 way z/VM host with a 64 way guest. It turned out that the guest was consuming all available cpus mostly for spinning on scheduler locks like rq->lock. This is expected as the threads are calling yield all the time. The problem is now, that the host scheduling decisings together with the guest scheduling decisions and spinlocks not being fair managed to create an interesting scenario similar to a live lock. (Sometimes the hang resolved itself after some minutes) Changing stop_machine to yield the cpu to the hypervisor when yielding inside the guest fixed the problem for me. While I am not completely happy with this patch, I think it causes no harm and it really improves the situation for me. I used cpu_relax for yielding to the hypervisor, does that work on all architectures? p.s.: If you want to reproduce the problem, cpu hotplug and kprobes use stop_machine_run and both triggered the problem after some retries. Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> CC: Ingo Molnar <mingo@elte.hu> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
215 lines
4.9 KiB
C
215 lines
4.9 KiB
C
/* Copyright 2005 Rusty Russell rusty@rustcorp.com.au IBM Corporation.
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* GPL v2 and any later version.
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*/
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#include <linux/cpu.h>
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#include <linux/err.h>
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#include <linux/kthread.h>
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#include <linux/module.h>
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#include <linux/sched.h>
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#include <linux/stop_machine.h>
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#include <linux/syscalls.h>
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#include <linux/interrupt.h>
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#include <asm/atomic.h>
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#include <asm/uaccess.h>
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/* Since we effect priority and affinity (both of which are visible
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* to, and settable by outside processes) we do indirection via a
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* kthread. */
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/* Thread to stop each CPU in user context. */
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enum stopmachine_state {
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STOPMACHINE_WAIT,
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STOPMACHINE_PREPARE,
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STOPMACHINE_DISABLE_IRQ,
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STOPMACHINE_EXIT,
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};
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static enum stopmachine_state stopmachine_state;
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static unsigned int stopmachine_num_threads;
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static atomic_t stopmachine_thread_ack;
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static int stopmachine(void *cpu)
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{
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int irqs_disabled = 0;
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int prepared = 0;
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set_cpus_allowed_ptr(current, &cpumask_of_cpu((int)(long)cpu));
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/* Ack: we are alive */
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smp_mb(); /* Theoretically the ack = 0 might not be on this CPU yet. */
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atomic_inc(&stopmachine_thread_ack);
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/* Simple state machine */
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while (stopmachine_state != STOPMACHINE_EXIT) {
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if (stopmachine_state == STOPMACHINE_DISABLE_IRQ
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&& !irqs_disabled) {
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local_irq_disable();
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hard_irq_disable();
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irqs_disabled = 1;
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/* Ack: irqs disabled. */
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smp_mb(); /* Must read state first. */
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atomic_inc(&stopmachine_thread_ack);
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} else if (stopmachine_state == STOPMACHINE_PREPARE
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&& !prepared) {
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/* Everyone is in place, hold CPU. */
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preempt_disable();
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prepared = 1;
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smp_mb(); /* Must read state first. */
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atomic_inc(&stopmachine_thread_ack);
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}
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/* Yield in first stage: migration threads need to
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* help our sisters onto their CPUs. */
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if (!prepared && !irqs_disabled)
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yield();
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cpu_relax();
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}
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/* Ack: we are exiting. */
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smp_mb(); /* Must read state first. */
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atomic_inc(&stopmachine_thread_ack);
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if (irqs_disabled)
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local_irq_enable();
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if (prepared)
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preempt_enable();
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return 0;
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}
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/* Change the thread state */
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static void stopmachine_set_state(enum stopmachine_state state)
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{
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atomic_set(&stopmachine_thread_ack, 0);
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smp_wmb();
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stopmachine_state = state;
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while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads)
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cpu_relax();
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}
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static int stop_machine(void)
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{
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int i, ret = 0;
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atomic_set(&stopmachine_thread_ack, 0);
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stopmachine_num_threads = 0;
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stopmachine_state = STOPMACHINE_WAIT;
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for_each_online_cpu(i) {
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if (i == raw_smp_processor_id())
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continue;
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ret = kernel_thread(stopmachine, (void *)(long)i,CLONE_KERNEL);
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if (ret < 0)
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break;
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stopmachine_num_threads++;
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}
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/* Wait for them all to come to life. */
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while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads) {
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yield();
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cpu_relax();
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}
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/* If some failed, kill them all. */
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if (ret < 0) {
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stopmachine_set_state(STOPMACHINE_EXIT);
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return ret;
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}
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/* Now they are all started, make them hold the CPUs, ready. */
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preempt_disable();
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stopmachine_set_state(STOPMACHINE_PREPARE);
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/* Make them disable irqs. */
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local_irq_disable();
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hard_irq_disable();
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stopmachine_set_state(STOPMACHINE_DISABLE_IRQ);
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return 0;
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}
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static void restart_machine(void)
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{
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stopmachine_set_state(STOPMACHINE_EXIT);
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local_irq_enable();
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preempt_enable_no_resched();
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}
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struct stop_machine_data {
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int (*fn)(void *);
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void *data;
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struct completion done;
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};
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static int do_stop(void *_smdata)
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{
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struct stop_machine_data *smdata = _smdata;
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int ret;
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ret = stop_machine();
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if (ret == 0) {
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ret = smdata->fn(smdata->data);
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restart_machine();
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}
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/* We're done: you can kthread_stop us now */
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complete(&smdata->done);
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/* Wait for kthread_stop */
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set_current_state(TASK_INTERRUPTIBLE);
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while (!kthread_should_stop()) {
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schedule();
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set_current_state(TASK_INTERRUPTIBLE);
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}
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__set_current_state(TASK_RUNNING);
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return ret;
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}
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struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
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unsigned int cpu)
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{
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static DEFINE_MUTEX(stopmachine_mutex);
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struct stop_machine_data smdata;
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struct task_struct *p;
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smdata.fn = fn;
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smdata.data = data;
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init_completion(&smdata.done);
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mutex_lock(&stopmachine_mutex);
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/* If they don't care which CPU fn runs on, bind to any online one. */
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if (cpu == NR_CPUS)
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cpu = raw_smp_processor_id();
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p = kthread_create(do_stop, &smdata, "kstopmachine");
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if (!IS_ERR(p)) {
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struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
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/* One high-prio thread per cpu. We'll do this one. */
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sched_setscheduler(p, SCHED_FIFO, ¶m);
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kthread_bind(p, cpu);
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wake_up_process(p);
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wait_for_completion(&smdata.done);
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}
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mutex_unlock(&stopmachine_mutex);
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return p;
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}
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int stop_machine_run(int (*fn)(void *), void *data, unsigned int cpu)
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{
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struct task_struct *p;
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int ret;
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/* No CPUs can come up or down during this. */
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get_online_cpus();
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p = __stop_machine_run(fn, data, cpu);
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if (!IS_ERR(p))
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ret = kthread_stop(p);
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else
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ret = PTR_ERR(p);
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put_online_cpus();
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return ret;
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}
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EXPORT_SYMBOL_GPL(stop_machine_run);
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