Re: [PATCH] stopmachine: add stopmachine_timeout v3

Peter Zijlstra
2008-07-16T07:33:23Z

On Wed, 2008-07-16 at 15:51 +0900, Hidetoshi Seto wrote:
> If stop_machine() invoked while one of onlined cpu is locked up
> by some reason, stop_machine cannot finish its work because the
> locked cpu cannot stop. This means all other healthy cpus
> will be blocked infinitely by one dead cpu.
>
> This patch allows stop_machine to return -EBUSY with some printk
> messages if any of stop_machine's threads cannot start running on
> its target cpu in time. You can enable this timeout via sysctl.
>
> v3:
> - set stopmachine_timeout default to 0 (= never timeout)
>
> v2:
> - remove fix for warning since it will be fixed upcoming typesafe
> patches
> - make stopmachine_timeout from secs to msecs
> - allow disabling timeout by setting the stopmachine_timeout to 0
>
> Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>

I really don't like this, it means the system is really screwed up and
doesn't deserve to continue.

> ---
> kernel/stop_machine.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++--
> kernel/sysctl.c | 15 +++++++++++++
> 2 files changed, 66 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 5b72c2b..77b7944 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -35,15 +35,18 @@ struct stop_machine_data {
> };
>
> /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
> -static unsigned int num_threads;
> +static atomic_t num_threads;
> static atomic_t thread_ack;
> +static cpumask_t prepared_cpus;
> static struct completion finished;
> static DEFINE_MUTEX(lock);
>
> +unsigned long stopmachine_timeout = 0; /* msecs, 0 = "never timeout" */
> +
> static void set_state(enum stopmachine_state newstate)
> {
> /* Reset ack counter. */
> - atomic_set(&thread_ack, num_threads);
> + atomic_set(&thread_ack, atomic_read(&num_threads));
> smp_wmb();
> state = newstate;
> }
> @@ -67,6 +70,8 @@ static int stop_cpu(struct stop_machine_data *smdata)
> enum stopmachine_state curstate = STOPMACHINE_NONE;
> int uninitialized_var(ret);
>
> + cpu_set(smp_processor_id(), prepared_cpus);
> +
> /* Simple state machine */
> do {
> /* Chill out and ensure we re-read stopmachine_state. */
> @@ -90,6 +95,7 @@ static int stop_cpu(struct stop_machine_data *smdata)
> }
> } while (curstate != STOPMACHINE_EXIT);
>
> + atomic_dec(&num_threads);
> local_irq_enable();
> do_exit(0);
> }
> @@ -105,6 +111,15 @@ int __stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
> int i, err;
> struct stop_machine_data active, idle;
> struct task_struct **threads;
> + unsigned long limit;
> +
> + if (atomic_read(&num_threads)) {
> + /*
> + * previous stop_machine was timeout, and still there are some
> + * unfinished thread (dangling stucked CPU?).
> + */
> + return -EBUSY;
> + }
>
> active.fn = fn;
> active.data = data;
> @@ -120,7 +135,7 @@ int __stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
> /* Set up initial state. */
> mutex_lock(&lock);
> init_completion(&finished);
> - num_threads = num_online_cpus();
> + atomic_set(&num_threads, num_online_cpus());
> set_state(STOPMACHINE_PREPARE);
>
> for_each_online_cpu(i) {
> @@ -152,10 +167,21 @@ int __stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
>
> /* We've created all the threads. Wake them all: hold this CPU so one
> * doesn't hit this CPU until we're ready. */
> + cpus_clear(prepared_cpus);
> get_cpu();
> for_each_online_cpu(i)
> wake_up_process(threads[i]);
>
> + /* Wait all others come to life */
> + if (stopmachine_timeout) {
> + limit = jiffies + msecs_to_jiffies(stopmachine_timeout);
> + while (cpus_weight(prepared_cpus) != num_online_cpus() - 1) {
> + if (time_is_before_jiffies(limit))
> + goto timeout;
> + cpu_relax();
> + }
> + }
> +
> /* This will release the thread on our CPU. */
> put_cpu();
> wait_for_completion(&finished);
> @@ -169,10 +195,32 @@ kill_threads:
> for_each_online_cpu(i)
> if (threads[i])
> kthread_stop(threads[i]);
> + atomic_set(&num_threads, 0);
> mutex_unlock(&lock);
>
> kfree(threads);
> return err;
> +
> +timeout:
> + printk(KERN_CRIT "stopmachine: Failed to stop machine in time(%lds).\n",
> + stopmachine_timeout);
> + for_each_online_cpu(i) {
> + if (!cpu_isset(i, prepared_cpus) && i != smp_processor_id())
> + printk(KERN_CRIT "stopmachine: cpu#%d seems to be "
> + "stuck.\n", i);
> + /* Unbind threads */
> + set_cpus_allowed(threads[i], cpu_online_map);
> + }
> +
> + /* Let threads go exit */
> + set_state(STOPMACHINE_EXIT);
> +
> + put_cpu();
> + /* no wait for completion */
> + mutex_unlock(&lock);
> + kfree(threads);
> +
> + return -EBUSY; /* canceled */
> }
>
> int stop_machine_run(int (*fn)(void *), void *data, const cpumask_t *cpus)
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 2911665..3c7ca98 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -146,6 +146,10 @@ extern int no_unaligned_warning;
> extern int max_lock_depth;
> #endif
>
> +#ifdef CONFIG_STOP_MACHINE
> +extern unsigned long stopmachine_timeout;
> +#endif
> +
> #ifdef CONFIG_PROC_SYSCTL
> static int proc_do_cad_pid(struct ctl_table *table, int write, struct file *filp,
> void __user *buffer, size_t *lenp, loff_t *ppos);
> @@ -813,6 +817,17 @@ static struct ctl_table kern_table[] = {
> .child = key_sysctls,
> },
> #endif
> +#ifdef CONFIG_STOP_MACHINE
> + {
> + .ctl_name = CTL_UNNUMBERED,
> + .procname = "stopmachine_timeout",
> + .data = &stopmachine_timeout,
> + .maxlen = sizeof(unsigned long),
> + .mode = 0644,
> + .proc_handler = &proc_doulongvec_minmax,
> + .strategy = &sysctl_intvec,
> + },
> +#endif
> /*
> * NOTE: do not add new entries to this table unless you have read
> * Documentation/sysctl/ctl_unnumbered.txt