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Date: Mon, 29 Apr 2013 22:27:03 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: David VomLehn <David.VomLehn@...cex.com>
Cc: linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] Msleep_interruptible() on a dual processor system may
wait a long time.
This should have been Cc'd to the scheduler maintainers.
-- Steve
On Mon, Apr 29, 2013 at 06:20:28PM -0700, David VomLehn wrote:
> Msleep_interruptible() on a dual processor system may wait a long time.
>
> On some reboots, calling msleep_interruptible() from CPU 1 on a dual
> processor system will not return for seconds or even minutes. This happens
> because ksoftirqd/1 migrates to CPU 0, which is allowed because its
> cpus_allowed mask is 0x3. Since ksoftirqd daemons only process the timer queue
> for their current CPU, no timer_list entries will be processed on CPU 1 until
> the ksoftirqd/1 migrates back to that CPU, which depends on system load and
> may take an arbitrary amount of time. The task associated with the
> msleep_interruptible() call may thus hang quite a while.
>
> The root cause appears to be to a race condition between select_fallback_rq(),
> which selects a runqueue for a task, and set_cpu_active(), which sets the
> corresponding bit in cpu_active_mask for a newly active CPU. When ksoftirqd/1
> is run for the first time, its cpus_allowed mask is set to 0x2, i.e. it is
> restricted to CPU 1. The function select_task_rq() will be called, which calls
> select_task_rq_fair(). This will return a 0 for the CPU on which to run the
> task. When select_task_rq() finds the task is not allowed to run on CPU 0,
> it calls select_fallback_rq() to choose a new CPU. There are two cases:
>
> o If set_cpu_active() ran for CPU 1 before select_fallback_rq(), the
> corresponding bit in cpu_active_mask will be set, allowing ksoftirqd/1
> to run on that CPU.
> o If the order of calls was reversed, select_fallback_rq() will call
> cpuset_cpus_allowed_fallback(), which will replace the task's
> cpus_allowed_mask with cpu_possible_mask, allowing ksoftirqd/1 to
> run on any CPU. It will also choose any CPU from the active CPUs.
>
> In the second case, ksoftirqd/1 will be able to roam freely across the
> system's CPUs, neglecting its responsibility to the timer queue.
>
> Signed-off-by: David VomLehn <dvomlehn@...cex.com>
> ---
> include/linux/cpu.h | 4 ++++
> include/linux/smp.h | 3 +++
> kernel/cpu.c | 5 +----
> kernel/sched.c | 4 ++++
> kernel/smp.c | 22 ++++++++++++++++++++--
> 5 files changed, 32 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index c692acc..9679dfe 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -138,6 +138,7 @@ int cpu_up(unsigned int cpu);
> void notify_cpu_starting(unsigned int cpu);
> extern void cpu_maps_update_begin(void);
> extern void cpu_maps_update_done(void);
> +extern int cpu_notify(unsigned long val, void *v);
>
> #else /* CONFIG_SMP */
>
> @@ -160,6 +161,9 @@ static inline void cpu_maps_update_done(void)
> {
> }
>
> +static inline int cpu_notify(unsigned long val, void *v)
> +{
> +}
> #endif /* CONFIG_SMP */
> extern struct sysdev_class cpu_sysdev_class;
>
> diff --git a/include/linux/smp.h b/include/linux/smp.h
> index 8cc38d3..1065d73 100644
> --- a/include/linux/smp.h
> +++ b/include/linux/smp.h
> @@ -37,6 +37,9 @@ int smp_call_function_single(int cpuid, smp_call_func_t func, void *info,
> #include <linux/thread_info.h>
> #include <asm/smp.h>
>
> +/* Number of CPUs we kicked that we are waiting to become active */
> +extern atomic_t active_cpu_pending;
> +
> /*
> * main cross-CPU interfaces, handles INIT, TLB flush, STOP, etc.
> * (defined in asm header):
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 563f136..4a11f33 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -150,7 +150,7 @@ static int __cpu_notify(unsigned long val, void *v, int nr_to_call,
> return notifier_to_errno(ret);
> }
>
> -static int cpu_notify(unsigned long val, void *v)
> +int cpu_notify(unsigned long val, void *v)
> {
> return __cpu_notify(val, v, -1, NULL);
> }
> @@ -315,9 +315,6 @@ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
> goto out_notify;
> BUG_ON(!cpu_online(cpu));
>
> - /* Now call notifier in preparation. */
> - cpu_notify(CPU_ONLINE | mod, hcpu);
> -
> out_notify:
> if (ret != 0)
> __cpu_notify(CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
> diff --git a/kernel/sched.c b/kernel/sched.c
> index fcc893f..907d166 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2539,6 +2539,10 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
> int dest_cpu;
> const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(cpu));
>
> + /* Loop because preempt we may be disabled or in atomic context */
> + while (atomic_read(&active_cpu_pending) != 0)
> + ;
> +
> /* Look for allowed, online CPU in same node. */
> for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
> if (cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)))
> diff --git a/kernel/smp.c b/kernel/smp.c
> index db197d6..d9635af 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -42,6 +42,17 @@ struct call_single_queue {
>
> static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_single_queue, call_single_queue);
>
> +/*
> + * Declarations to support waiting until the cpu_up() functions are all
> + * called before trying to wake up associated softirqds. Note that
> + * active_cpu_pending is initialized to one to prevent waiters from
> + * trying to do things before we get to smp_init().
> + */
> +atomic_t active_cpu_pending = ATOMIC_INIT(1);
> +static __initdata DECLARE_BITMAP(cpu_notify_pending_bits, CONFIG_NR_CPUS);
> +struct __initdata cpumask * const cpu_notify_pending_mask =
> + to_cpumask(cpu_notify_pending_bits);
> +
> static int
> hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
> {
> @@ -673,10 +684,17 @@ void __init smp_init(void)
> for_each_present_cpu(cpu) {
> if (num_online_cpus() >= setup_max_cpus)
> break;
> - if (!cpu_online(cpu))
> - cpu_up(cpu);
> + if (!cpu_online(cpu) && cpu_up(cpu) == 0)
> + cpumask_set_cpu(cpu, cpu_notify_pending_mask);
> }
>
> + /* Release anyone waiting for active CPUs to be identified */
> + atomic_set(&active_cpu_pending, 0);
> +
> + /* Now call notifier in preparation. */
> + for_each_cpu(cpu, cpu_notify_pending_mask)
> + cpu_notify(CPU_ONLINE, (void *)(long)cpu);
> +
> /* Any cleanup work */
> printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus());
> smp_cpus_done(setup_max_cpus);
> --
> David VL
> --
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