sorry for spam, Adding list
On 4/30/2018 4:47 PM, Peter Zijlstra wrote:
On Thu, Apr 26, 2018 at 09:23:25PM +0530, Kohli, Gaurav wrote:
On 4/26/2018 2:27 PM, Peter Zijlstra wrote:
On Thu, Apr 26, 2018 at 10:41:31AM +0200, Peter Zijlstra wrote:
diff --git a/kernel/kthread.c b/kernel/kthread.c
index cd50e99202b0..4b6503c6a029 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -177,12 +177,13 @@ void *kthread_probe_data(struct task_struct *task)
static void __kthread_parkme(struct kthread *self)
{
- __set_current_state(TASK_PARKED);
- while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
+ for (;;) {
+ __set_task_state(TASK_PARKED);
set_current_state(TASK_PARKED);
of course..
Hi Peter,
Maybe i am missing something , but still that race can come as we don't put task_parked on special state.
Controller Hotplug
Loop
Task_Interruptible
Set SHOULD_PARK
wakeup -> Proceeds
Set Running
kthread_parkme
SET TASK_PARKED
schedule
Set TASK_RUNNING
Can you please correct ME, if I misunderstood this.
If that could happen, all wait-loops would be broken. However,
AFAICT that cannot happen, because ttwu_remote() and schedule()
serialize on rq->lock. See:
A B
for (;;) {
set_current_state(UNINTERRUPTIBLE);
cond1 = true;
wake_up_process(A)
lock(A->pi_lock)
smp_mb__after_spinlock()
if (A->state & TASK_NORMAL)
A->on_rq && ttwu_remote()
if (cond1) // true
break;
schedule();
}
__set_current_state(RUNNING);
Hi Peter,
Sorry for the late reply and i was on leave.
Thanks for the new patches, We will apply and test for issue reproduction.
But In our older case, where we have seen failure below is the wake up
path and ftraces, Wakeup occured and completed before schedule call only.
So final state of CPUHP is running not parked. I have also pasted debug
ftraces that we got during issue reproduction.
Here wakeup for cpuhp is below:
takedown_cpu-> kthread_park-> wake_up_process
39,034,311,742,395 apps (10240) Trace Printk
cpuhp/0 (16) [000] 39015.625000: <debug> __kthread_parkme state=512
task=ffffffcc7458e680 flags: 0x5 -> state 5 -> state is parked inside
parkme function
39,034,311,846,510 apps (10240) Trace Printk
cpuhp/0 (16) [000] 39015.625000: <debug> before schedule
__kthread_parkme state=0 task=ffffffcc7458e680 flags: 0xd -> just
before schedule call, state is running
tatic void __kthread_parkme(struct kthread *self)
{
__set_current_state(TASK_PARKED);
while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
complete(&self->parked);
schedule();
__set_current_state(TASK_PARKED);
}
clear_bit(KTHREAD_IS_PARKED, &self->flags);
__set_current_state(TASK_RUNNING);
}
So my point is here also, if it is reschedule then it can set
TASK_PARKED, but it seems after takedown_cpu call this thread never get
a chance to run, So final state is TASK_RUNNING.
In our current fix also can't we observe same scenario where final state
is TASK_RUNNING.
Regards
Gaurav
for (;;) {
set_current_state(UNINTERRUPTIBLE);
if (cond2)
break;
schedule();
lock(rq->lock)
smp_mb__after_spinlock();
deactivate_task(A);
<sched-out>
unlock(rq->lock);
rq = __task_rq_lock(A)
if (A->on_rq) // false
A->state = TASK_RUNNING;
__task_rq_unlock(rq)
Either A's schedule() must observe RUNNING (not shown) or B must
observe !A->on_rq (shown) and not issue the store.
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