Re: [PATCH] avoid cpu removal if busy revisited

["Randy.Dunlap" <rdunlap@xenotime.net>]

On Fri, 23 Jun 2006 10:50:58 +0900 KAMEZAWA Hiroyuki wrote:

> Hi, 
> thank you for good responses. This is avoid_cpu_removal_if_busy patch.
> 
> I added all folks to CC who replied previous "stop_on_cpu_lost" discussion.
> 
> How about this ?
> (my vocabulary is not rich, then please tell me if you know better name for
>  sysctl.)
> 
> Changes from old version
> - added sysctl
> 
> -Kame
> ==
> 
> Now, cpu hot remove migrates all tasks on target cpu by force.
> 
> During cpu-hot-remove, if tsk->cpus_allowed contains the only target
> cpu of removal, tsk->cpus_allowd is disposed and the kernel migrate it to
> random cpu.It's obvious that user-land configuration before cpu hot removal
> is bad. But this is not good in carefully scheduled environment.
> 
> In this case,
> 1. ignore bad configuration in user-land just do warnings.
> 2. cancel cpu hot removal and warn users to fix the problem and retry.
> seems to be a realisitc workaround. Killing the problematic process may
> cause some trouble in user-land (dead-lock etc..)
> 
> This patch adds sysctl moderate_cpu_removal.
> If moderate_cpu_removal == 0, all tasks are migrated by force.
> If moderate_cpu_removal == 1, cpu_hotremoval can fail because of not-
> migratable tasks.


Maybe cpu_removal_migrate?  I think that inverts the sysctl
values though, like so:

If cpu_removal_migrate == 1, all tasks are migrated by force.
If cpu_removal_migrate == 0, cpu_hotremoval can fail because of
not-migratable tasks (tasks bound to the target CPU).

and init. the sysctl value to 1 as default.


> Signed-Off-By: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> 
> 
>  include/linux/sysctl.h |    1 +
>  kernel/sched.c         |   42 ++++++++++++++++++++++++++++++++++++++++++
>  kernel/sysctl.c        |   13 +++++++++++++
>  3 files changed, 56 insertions(+)
> 
> Index: linux-2.6.17.org/kernel/sched.c
> ===================================================================
> --- linux-2.6.17.org.orig/kernel/sched.c
> +++ linux-2.6.17.org/kernel/sched.c
> @@ -4562,6 +4562,44 @@ wait_to_die:
>  }
>  
>  #ifdef CONFIG_HOTPLUG_CPU
> +/*
> + * if moderate_cpu_removal==1 (sysctl), cpu-hot-remove will fail if cpu is busy.
> + * Default value is 0. all tasks are forced to migrate.
> + */
> +int moderate_cpu_removal;

This is also declared (defined?  I get those mixed up) in
kernel/sysctl.c.  One of them (this one I think) should be
extern, but we prefer externs in a header file if possible.


> +
> +/*
> + * test there are tasks tightly coupled to the target cpu.
> + */
> +static int test_cpu_busy(int cpu)
> +{
> +	cpumask_t mask;
> +	int ret = 0;
> +	pid_t pid;
> +	struct task_struct *p;
> +	cpus_clear(mask);
> +	cpu_set(cpu, mask);
> +
> +	read_lock(&tasklist_lock);
> +	for_each_process(p) {
> +		if (p == current)
> +			continue;
> +		if (p->mm && cpus_equal(mask, p->cpus_allowed)) {
> +			ret = 1;
> +			pid = p->pid;
> +			break;
> +		}
> +	}
> +	read_unlock(&tasklist_lock);
> +	if (ret) {
> +		printk(KERN_ERR "cpu(%d) is busy because of task(%d)\n",
> +			cpu, pid);
> +		printk(KERN_ERR "adjust task(%d) configuration or set "
> +			"moderate_cpu_removal to 0 to remove cpu %d\n",
> +			pid, cpu);
> +	}
> +	return ret;
> +}
>  /* Figure out where task on dead CPU should go, use force if neccessary. */
>  static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *tsk)
>  {
> @@ -4752,6 +4790,10 @@ static int migration_call(struct notifie
>  		kthread_stop(cpu_rq(cpu)->migration_thread);
>  		cpu_rq(cpu)->migration_thread = NULL;
>  		break;
> +	case CPU_DOWN_PREPARE:
> +		if (moderate_cpu_removal && test_cpu_bust(cpu))
> +			return NOTIFY_BAD;
> +		break;
>  	case CPU_DEAD:
>  		migrate_live_tasks(cpu);
>  		rq = cpu_rq(cpu);
> Index: linux-2.6.17.org/kernel/sysctl.c
> ===================================================================
> --- linux-2.6.17.org.orig/kernel/sysctl.c
> +++ linux-2.6.17.org/kernel/sysctl.c
> @@ -78,6 +78,9 @@ int unknown_nmi_panic;
>  extern int proc_unknown_nmi_panic(ctl_table *, int, struct file *,
>  				  void __user *, size_t *, loff_t *);
>  #endif
> +#ifdef CONFIG_HOTPLUG_CPU
> +int moderate_cpu_removal;
> +#endif
>  
>  /* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
>  static int maxolduid = 65535;
> @@ -683,6 +686,16 @@ static ctl_table kern_table[] = {
>  		.proc_handler	= &proc_dointvec,
>  	},
>  #endif
> +#ifdef CONFIG_HOTPLUG_CPU
> +	{
> +		.ctl_name	= KERN_MODERATE_CPU_REMOVAL,
> +		.procname	= "moderate_cpu_removal",
> +		.data		= &moderate_cpu_removal,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= &proc_dointvec,
> +	}
> +#endif
>  	{ .ctl_name = 0 }
>  };
>  
> Index: linux-2.6.17.org/include/linux/sysctl.h
> ===================================================================
> --- linux-2.6.17.org.orig/include/linux/sysctl.h
> +++ linux-2.6.17.org/include/linux/sysctl.h
> @@ -148,6 +148,7 @@ enum
>  	KERN_SPIN_RETRY=70,	/* int: number of spinlock retries */
>  	KERN_ACPI_VIDEO_FLAGS=71, /* int: flags for setting up video after ACPI sleep */
>  	KERN_IA64_UNALIGNED=72, /* int: ia64 unaligned userland trap enable */
> +	KERN_MODERATE_CPU_REMOVAL=73, /* int: disallow forced cpu removal */
>  };


---
~Randy