Re: + cpu_hotplug-dont-affect-current-tasks-affinity.patch added to -mm tree

Oleg Nesterov
2009-07-29T02:36:54Z

Noticed this patch by accident...

> Subject: cpu_hotplug: don't affect current task's affinity
> From: Lai Jiangshan <laijs@cn.fujitsu.com>
>
> _cpu_down() changes the current task's affinity and then recovers it at
> the end.
>
> It has two problems:

Yes.

But can't we make a much more simple patch ?

Why should we play with set_cpus_allowed() at all? We can just migrate
the caller of _cpu_down() before other tasks, right after take_cpu_down()
does __cpu_disable()->remove_cpu_from_maps().

No?

Oleg.

------------------------------------------------------------------------
Uncompiled, and we need to export move_task_off_dead_cpu(). Just to
explain what I mean.

kernel/cpu.c | 18 +++++-------------
1 file changed, 5 insertions(+), 13 deletions(-)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -163,6 +163,7 @@ static inline void check_for_tasks(int c
}

struct take_cpu_down_param {
+ struct task_struct *caller;
unsigned long mod;
void *hcpu;
};
@@ -181,6 +182,8 @@ static int __ref take_cpu_down(void *_pa
raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
param->hcpu);

+ move_task_off_dead_cpu(param->hcpu, param->caller);
+
/* Force idle task to run as soon as we yield: it should
immediately notice cpu is offline and die quickly. */
sched_idle_next();
@@ -191,10 +194,10 @@ static int __ref take_cpu_down(void *_pa
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
int err, nr_calls = 0;
- cpumask_var_t old_allowed;
void *hcpu = (void *)(long)cpu;
unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
struct take_cpu_down_param tcd_param = {
+ .caller = current,
.mod = mod,
.hcpu = hcpu,
};
@@ -205,9 +208,6 @@ static int __ref _cpu_down(unsigned int
if (!cpu_online(cpu))
return -EINVAL;

- if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
- return -ENOMEM;
-
cpu_hotplug_begin();
err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
hcpu, -1, &nr_calls);
@@ -221,11 +221,6 @@ static int __ref _cpu_down(unsigned int
goto out_release;
}

- /* Ensure that we are not runnable on dying cpu */
- cpumask_copy(old_allowed, &current->cpus_allowed);
- set_cpus_allowed_ptr(current,
- cpumask_of(cpumask_any_but(cpu_online_mask, cpu)));
-
err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
if (err) {
/* CPU didn't die: tell everyone. Can't complain. */
@@ -233,7 +228,7 @@ static int __ref _cpu_down(unsigned int
hcpu) == NOTIFY_BAD)
BUG();

- goto out_allowed;
+ goto out_release;
}
BUG_ON(cpu_online(cpu));

@@ -251,8 +246,6 @@ static int __ref _cpu_down(unsigned int

check_for_tasks(cpu);

-out_allowed:
- set_cpus_allowed_ptr(current, old_allowed);
out_release:
cpu_hotplug_done();
if (!err) {
@@ -260,7 +253,6 @@ out_release:
hcpu) == NOTIFY_BAD)
BUG();
}
- free_cpumask_var(old_allowed);
return err;
}