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SubjectRe: current linux-2.6.git: cpusets completely broken

On Sat, 12 Jul 2008, Linus Torvalds wrote:
> > Being called for CPU_UP_PREPARE (and if its callback is called after
> > update_sched_domains()), it just negates all the work done by
> > update_sched_domains() -- i.e. a soon-to-be-offline cpu is included in
> > the sched-domains and that makes it visible for the load-balancer
> > while the CPU_DOWN ops. is in progress.
> This sounds like it could trigger various other problems too, but happily
> hit the BUG_ON() first.

Btw - the way to avoid this whole problem might be to make CPU migration
use a *different* CPU map than "online".

This patch almost certainly doesn't work, but let me explain:

- "cpu_online_map" is the CPU's that can be currently be running

It is enabled/disabled by low-level architecture code when the CPU
actually gets disabled.

- Add a new "cpu_active_map", which is the CPU's that are currently fully
set up, and can not just be running tasks, but can be _migrated_ to!

- We can always just clear the "cpu_active_map" entry when we start a CPU
down event - that guarantees that while tasks may be running on it,
there won't be any _new_ tasks migrated to it.

- both cpu_down() and cpu_up() can just end with a simple

if (cpu_online(cpu))
cpu_set(cpu, cpu_active_map);

before they release the hotplug lock, and it will always do the right
thing regardless of whether the up/down succeeded or not.

The _only_ thing that "active" is used for is literally to verify that a
migration is valid before doing it.

Now, a few points:
(a) it's *TOTALLY* untested. It may or may not compile.
(b) I think many architectures do magic things at the initial boot, and
change the online maps in odd ways. I tried to avoid this by simply
doing the initial "cpu_set()" of cpu_active_map pretty late, just
before we bring up other CPUs
(c) I think this is a pretty simple approach - and like how all the code
is architecture-neutral. The "active" map may not be used a lot, but
doesn't this simplify the whole problem a lot? It just makes the
whole scheduling issue go away for CPU's that are going down.

What do you guys think? Ingo?

Vegard, and just out of interest, in case this would happen to work, does
this actually end up also fixing the bug (with the other fixes unapplied?)


include/linux/cpumask.h | 15 ++++++++++++++-
init/main.c | 7 +++++++
kernel/cpu.c | 8 +++++++-
kernel/sched.c | 2 +-
4 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index c24875b..88f2dd2 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -359,13 +359,14 @@ static inline void __cpus_fold(cpumask_t *dstp, const cpumask_t *origp,

* The following particular system cpumasks and operations manage
- * possible, present and online cpus. Each of them is a fixed size
+ * possible, present, active and online cpus. Each of them is a fixed size
* bitmap of size NR_CPUS.
* cpu_possible_map - has bit 'cpu' set iff cpu is populatable
* cpu_present_map - has bit 'cpu' set iff cpu is populated
* cpu_online_map - has bit 'cpu' set iff cpu available to scheduler
+ * cpu_active_map - has bit 'cpu' set iff cpu available to migration
* #else
* cpu_possible_map - has bit 'cpu' set iff cpu is populated
* cpu_present_map - copy of cpu_possible_map
@@ -417,6 +418,16 @@ extern cpumask_t cpu_possible_map;
extern cpumask_t cpu_online_map;
extern cpumask_t cpu_present_map;

+ * With CONFIG_HOTPLUG_CPU, cpu_active_map is a real instance.
+ * Without hotplugging, "online" and "active" are the same.
+ */
+extern cpumask_t cpu_active_map;
+#define cpu_active_map cpu_online_map
#if NR_CPUS > 1
#define num_online_cpus() cpus_weight(cpu_online_map)
#define num_possible_cpus() cpus_weight(cpu_possible_map)
@@ -424,6 +435,7 @@ extern cpumask_t cpu_present_map;
#define cpu_online(cpu) cpu_isset((cpu), cpu_online_map)
#define cpu_possible(cpu) cpu_isset((cpu), cpu_possible_map)
#define cpu_present(cpu) cpu_isset((cpu), cpu_present_map)
+#define cpu_active(cpu) cpu_isset((cpu), cpu_active_map)
#define num_online_cpus() 1
#define num_possible_cpus() 1
@@ -431,6 +443,7 @@ extern cpumask_t cpu_present_map;
#define cpu_online(cpu) ((cpu) == 0)
#define cpu_possible(cpu) ((cpu) == 0)
#define cpu_present(cpu) ((cpu) == 0)
+#define cpu_active(cpu) ((cpu) == 0)

#define cpu_is_offline(cpu) unlikely(!cpu_online(cpu))
diff --git a/init/main.c b/init/main.c
index f7fb200..bfccff6 100644
--- a/init/main.c
+++ b/init/main.c
@@ -414,6 +414,13 @@ static void __init smp_init(void)
unsigned int cpu;

+ /*
+ * Set up the current CPU as possible to migrate to.
+ * The other ones will be done by cpu_up/cpu_down()
+ */
+ cpu = smp_processor_id();
+ cpu_set(cpu, cpu_active_map);
/* FIXME: This should be done in userspace --RR */
for_each_present_cpu(cpu) {
if (num_online_cpus() >= setup_max_cpus)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index c77bc3a..2a30026 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -44,6 +44,8 @@ void __init cpu_hotplug_init(void)


+cpumask_t cpu_active_map;
void get_online_cpus(void)
@@ -269,11 +271,13 @@ int __ref cpu_down(unsigned int cpu)
int err = 0;

+ cpu_clear(cpu, cpu_active_map);
if (cpu_hotplug_disabled)
err = -EBUSY;
err = _cpu_down(cpu, 0);
+ if (cpu_online(cpu))
+ cpu_set(cpu, cpu_active_map);
return err;
@@ -337,6 +341,8 @@ int __cpuinit cpu_up(unsigned int cpu)
err = _cpu_up(cpu, 0);

+ if (cpu_online(cpu))
+ cpu_set(cpu, cpu_active_map);
return err;
diff --git a/kernel/sched.c b/kernel/sched.c
index 4e2f603..21ee025 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2680,7 +2680,7 @@ static void sched_migrate_task(struct task_struct *p, int dest_cpu)

rq = task_rq_lock(p, &flags);
if (!cpu_isset(dest_cpu, p->cpus_allowed)
- || unlikely(cpu_is_offline(dest_cpu)))
+ || unlikely(!cpu_active(dest_cpu)))
goto out;

/* force the process onto the specified CPU */

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