Re: CPU Hotplug broken -mm5 onwards

From: Srivatsa Vaddagiri
Date: Wed Apr 21 2004 - 11:57:15 EST

On Wed, Apr 21, 2004 at 03:29:39PM +0530, Srivatsa Vaddagiri wrote:
> On Wed, Apr 21, 2004 at 02:36:50AM -0700, Andrew Morton wrote:
> >
> > Not sure who to aim this at, but: poke.
> I found that removing the lock in exec path has other consequences
> which I am fixing/testing. I will be sending an updated patch later today
> (against 2.6.6-rc1-mm1) to fix all those issues.

Found that lockless migration of execing task is _extremely_ racy.
The races I hit are described below, alongwith probable solutions.

Task migration done elsewhere should be safe (?) since they either
hold the lock (sys_sched_setaffinity) or are done entirely with preemption
disabled (load_balance).

sched_balance_exec does:

a. disables preemption
b. finds new_cpu for current
c. enables preemption
d. calls sched_migrate_task to migrate current to new_cpu

and sched_migrate_task does:

e. task_rq_lock(p)
f. migrate_task(p, dest_cpu ..)
(if we have to wait for migration thread)
g. task_rq_unlock()
h. wake_up_process(rq->migration_thread)
i. wait_for_completion()

Several things can happen here:

1. new_cpu can go down after h and before migration thread has
got around to handle the request

==> we need to add a cpu_is_offline check in __migrate_task

2. new_cpu can go down between c and d or before f.

===> Even though this case is automatically handled by the above
change (migrate_task being called on a running task, current,
will delegate migration to migration thread), would it be
good practice to avoid calling migrate_task in the first place
itself when dest_cpu is offline. This means adding another
cpu_is_offline check after e in sched_migrate_task

3. The 'current' task can get preempted _immediately_ after
g and when it comes back, task_cpu(p) can be dead. In
which case, it is invalid to do wake_up on a non-existent migration
thread. (rq->migration_thread can be NULL).

===> We should disable preemption thr' g and h

4. Before migration thread gets around to handle the request, its cpu
goes dead. This will leave unhandled migration requests in the dead

===> We need to wakeup sleeping requestors (if any) in CPU_DEAD

I really wonder if we can get rid of these issues by avoiding balancing at
exec time and instead have it balanced during load_balance ..Alternately
if this is valuable and we want to retain it, I think we still need to
consider a read/write sem, with sched_migrate_task doing down_read_trylock.
This may eliminate the deadlock I hit between cpu_up and CPU_UP_PREPARE
notification, which had forced me away from r/w sem.

Anyway patch below addresses the above races. Its against 2.6.6-rc2-mm1
and has been tested on a 4way Intel Pentium SMP m/c. I have added a
cpu_is_offlince check in migration_thread(). If that is true, migration
thread stop processing and just waits for kthread_stop.

Let me know what you think!


linux-2.6.6-rc2-mm1-root/kernel/sched.c | 40 ++++++++++++++++++++++++++++----
1 files changed, 36 insertions(+), 4 deletions(-)

diff -puN kernel/sched.c~remove_hotplug_lock_in_sched_migrate_task kernel/sched.c
--- linux-2.6.6-rc2-mm1/kernel/sched.c~remove_hotplug_lock_in_sched_migrate_task 2004-04-20 20:42:03.000000000 +0530
+++ linux-2.6.6-rc2-mm1-root/kernel/sched.c 2004-04-20 22:22:16.527816944 +0530
@@ -1420,16 +1420,18 @@ static void sched_migrate_task(task_t *p
runqueue_t *rq;
unsigned long flags;

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

/* force the process onto the specified CPU */
if (migrate_task(p, dest_cpu, &req)) {
/* Need to wait for migration thread. */
+ preempt_disable();
task_rq_unlock(rq, &flags);
+ preempt_enable();

@@ -1438,13 +1440,11 @@ static void sched_migrate_task(task_t *p
* the migration.
- unlock_cpu_hotplug();

task_rq_unlock(rq, &flags);
- unlock_cpu_hotplug();

@@ -3485,6 +3485,9 @@ static void __migrate_task(struct task_s
runqueue_t *rq_dest;

+ if (unlikely(cpu_is_offline(dest_cpu)))
+ return;
rq_dest = cpu_rq(dest_cpu);

double_rq_lock(this_rq(), rq_dest);
@@ -3540,6 +3543,8 @@ static int migration_thread(void * data)
if (list_empty(head)) {
+ if (unlikely(cpu_is_offline(cpu)))
+ goto wait_to_die;
req = list_entry(head->next, migration_req_t, list);
@@ -3560,6 +3565,15 @@ static int migration_thread(void * data)
return 0;
+ /* Wait for kthread_stop */
+ set_current_state(TASK_INTERRUPTIBLE);
+ while (!kthread_should_stop()) {
+ schedule();
+ set_current_state(TASK_INTERRUPTIBLE);
+ }
+ __set_current_state(TASK_RUNNING);
+ return 0;

@@ -3630,6 +3644,8 @@ static int migration_call(struct notifie
struct task_struct *p;
struct runqueue *rq;
unsigned long flags;
+ struct list_head *head;
+ migration_req_t *req, *tmp;

switch (action) {
@@ -3652,6 +3668,22 @@ static int migration_call(struct notifie
/* Unbind it from offline cpu so it can run. Fall thru. */
case CPU_DEAD:
+ rq = cpu_rq(cpu);
+ head = &rq->migration_queue;
+ spin_lock_irq(&rq->lock);
+ list_for_each_entry_safe(req, tmp, head, list) {
+ BUG_ON(req->type != REQ_MOVE_TASK);
+ list_del_init(&req->list);
+ /* No need to migrate the task in the request. It would
+ * have already been migrated (maybe to a different
+ * CPU). Just wake up the requestor.
+ */
+ complete(&req->done);
+ }
+ spin_unlock_irq(&rq->lock);
cpu_rq(cpu)->migration_thread = NULL;
BUG_ON(cpu_rq(cpu)->nr_running != 0);


The above patch is running fine against 2.6.6-rc2-mm1 for the last 3 hrs.

The same patch when I had tried against 2.6.6-rc1-mm1 lead to
this BUG in include/asm-i386/mmu_context.h after running for 1 hr.

BUG_ON(cpu_tlbstate[cpu].active_mm != next);

Stack trace was:

------------[ cut here ]------------
kernel BUG at include/asm/mmu_context.h:53!
invalid operand: 0000 [#1]
CPU: 0
EIP: 0060:[<c03c94a2>] Not tainted VLI
EFLAGS: 00010087 (2.6.6-rc1-mm1)
EIP is at schedule+0x482/0x62d
eax: 00000000 ebx: d34a0a00 ecx: 00000000 edx: f73f1a80
esi: d01b2b70 edi: c170bca0 ebp: ebcbbfb0 esp: ebcbbf5c
ds: 007b es: 007b ss: 0068
Process kstopmachine (pid: 19332, threadinfo=ebcba000 task=d34a0850)
Stack: d34a0850 c170bca0 00000001 ebcbbf8c f73f1a80 d01b2b70 00000003 00000000
cad73ee8 ebcba000 cad73ee0 00000292 d01b2b70 00002fcb c04c53d9 0000037b
d34a0850 d34a0a00 ebcba000 fffffff0 cad73ed8 c013bd3f c013bd7b 00000000
Call Trace:
[<c013bd3f>] do_stop+0x0/0x6f
[<c013bd7b>] do_stop+0x3c/0x6f
[<c0133d39>] kthread+0xb7/0xbd
[<c0133c82>] kthread+0x0/0xbd
[<c01023a1>] kernel_thread_helper+0x5/0xb

Don't know if it is related to something that is fixed in 2.6.6-rc2-mm1.

Will update this list tomorrow with how my tests are faring on 2.6.6-rc2-mm1.


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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