Re: [PATCH][RFC] kernel/cpu: do not change the cpus_allowed of thecurrent task when unplugging cpus

From: Sebastian Andrzej Siewior
Date: Fri Jun 14 2013 - 11:29:23 EST

* Zhao Chenhui | 2013-06-09 17:59:42 [+0800]:

>No. _cpu_down() on mainline do not change the cpus_allowed.
My bad.

>The problem is that the task which turned off cpu2 (for instance)
>can not run on cpu2 again after cpu2 is turned on, because cpu2 has been
>removed from the cpus_allowed of the task.
>The task can put himself back on cpu2 throuhg the system call,
>but I think applications should not do this work and do not care which cpu
>it is running on in most time.

The mask needs to be changed because you may not be on the CPU while it
is going down. What do you think about:

Subject: [PATCH] kernel/hotplug: restore original cpu mask oncpu/down

If a task which is allowed to run only on CPU X puts CPU Y down then it
will be allowed on all CPUs but the on CPU Y after it comes back from
kernel. This patch ensures that we don't lose the initial setting unless
the CPU the task is running is going down.

Cc: stable-rt@xxxxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
kernel/cpu.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 3acf17d..f5ad8e1 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -545,6 +545,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
.hcpu = hcpu,
cpumask_var_t cpumask;
+ cpumask_var_t cpumask_org;

if (num_online_cpus() == 1)
return -EBUSY;
@@ -555,6 +556,12 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
/* Move the downtaker off the unplug cpu */
if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
return -ENOMEM;
+ if (!alloc_cpumask_var(&cpumask_org, GFP_KERNEL)) {
+ free_cpumask_var(cpumask);
+ return -ENOMEM;
+ }
+ cpumask_copy(cpumask_org, tsk_cpus_allowed(current));
cpumask_andnot(cpumask, cpu_online_mask, cpumask_of(cpu));
set_cpus_allowed_ptr(current, cpumask);
@@ -563,7 +570,8 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
if (mycpu == cpu) {
printk(KERN_ERR "Yuck! Still on unplug CPU\n!");
- return -EBUSY;
+ err = -EBUSY;
+ goto restore_cpus;

@@ -622,6 +630,9 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
if (!err)
cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
+ set_cpus_allowed_ptr(current, cpumask_org);
+ free_cpumask_var(cpumask_org);
return err;



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