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SubjectRe: [RFC][PATCH] avoid cpu hot remove of cpus which have special RT tasks.
On Fri, 16 Jun 2006 09:09:22 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> On Fri, 16 Jun 2006, KAMEZAWA Hiroyuki wrote:
>
> > When cpu hot remove happens, tasks on the target cpu will be migrated even if
> > no available cpus in tsk->cpus_allowed. (See: move_task_off_dead_cpu().)
>
> Could we kill the process instead? If a process has been forced to run on
> a certain cpu then it is an error to migrate it to a different one. If a
> system wiil do cpu hot remove then the system needs to be configured in
> such a way as to allow processes to be migrated to other processors.
>
How about this ? SIGKILL is better ?
(I'm a bit afraid that this force_sig is safe or not...)

good night..
-Kame
===
This patch adds sysctl "stop derailed process".

If stop_derailed_process == 1, a process will be stopped by SIGSTOP
which has cpu affinity but is forced to migrate to an unexpected cpu .

This will prevent unexpected trouble in multi-threaded application whose threads
are tightly coupled to specified cpus.

Signed-Off-By: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

include/linux/sysctl.h | 1 +
kernel/sched.c | 11 +++++++++++
kernel/sysctl.c | 14 ++++++++++++++
3 files changed, 26 insertions(+)
Index: linux-2.6.17-rc6-mm2/kernel/sched.c
===================================================================
--- linux-2.6.17-rc6-mm2.orig/kernel/sched.c
+++ linux-2.6.17-rc6-mm2/kernel/sched.c
@@ -4953,11 +4953,16 @@ wait_to_die:
}

#ifdef CONFIG_HOTPLUG_CPU
-/* Figure out where task on dead CPU should go, use force if neccessary. */
+int stop_derailed_process;
+
+/* Figure out where task on dead CPU should go, use force if neccessary.
+ if stop_derailed_process sysctl is 1, processes migrated to unexpected
+ cpus will stop.*/
static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *tsk)
{
int dest_cpu;
cpumask_t mask;
+ int force = 0;

/* On same node? */
mask = node_to_cpumask(cpu_to_node(dead_cpu));
@@ -4982,8 +4987,16 @@ static void move_task_off_dead_cpu(int d
printk(KERN_INFO "process %d (%s) no "
"longer affine to cpu%d\n",
tsk->pid, tsk->comm, dead_cpu);
+ if (tsk->mm && stop_derailed_process) {
+ force = 1;
+ printk(KERN_INFO, "process %d (%s) is stopped "
+ "by stop_derailed_process sysctl\n",
+ tsk->pid, tsk->comm);
+ }
}
__migrate_task(tsk, dead_cpu, dest_cpu);
+ if (force)
+ force_sig_specific(SIGSTOP, tsk);
}

/*
Index: linux-2.6.17-rc6-mm2/kernel/sysctl.c
===================================================================
--- linux-2.6.17-rc6-mm2.orig/kernel/sysctl.c
+++ linux-2.6.17-rc6-mm2/kernel/sysctl.c
@@ -90,6 +90,10 @@ extern int proc_nmi_enabled(struct ctl_t
void __user *, size_t *, loff_t *);
#endif

+#ifdef CONFIG_HOTPLUG_CPU
+extern int stop_derailed_process;
+#endif
+
/* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
static int maxolduid = 65535;
static int minolduid;
@@ -784,6 +788,16 @@ static ctl_table kern_table[] = {
.proc_handler = &proc_dointvec,
},
#endif
+#ifdef CONFIG_HOTPLUG_CPU
+ {
+ .ctl_name = KERN_STOP_DERAILED_PROC,
+ .procname = "stop_derailed_process",
+ .data = &stop_derailed_process,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ }
+#endif

{ .ctl_name = 0 }
};
Index: linux-2.6.17-rc6-mm2/include/linux/sysctl.h
===================================================================
--- linux-2.6.17-rc6-mm2.orig/include/linux/sysctl.h
+++ linux-2.6.17-rc6-mm2/include/linux/sysctl.h
@@ -153,6 +153,7 @@ enum
KERN_NMI_WATCHDOG=74, /* int: enable/disable nmi watchdog */
KERN_PANIC_ON_NMI=75, /* int: whether we will panic on an unrecovered */
KERN_MAX_LOCK_DEPTH=76,
+ KERN_STOP_DERAILED_PROCESS=77, /* int: stop cpu bound process if the cpu is removed */
};


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