On 5/2/2018 1:50 PM, Peter Zijlstra wrote:
On Wed, May 02, 2018 at 10:45:52AM +0530, Kohli, Gaurav wrote:
On 5/1/2018 6:49 PM, Peter Zijlstra wrote:
- complete(&kthread->parked), which we can do inside schedule(); this
solves the problem because then kthread_park() will not return early
and the task really is blocked.
I think complete will not help, as problem is like below :
Control Thread CPUHP thread
cpuhp_thread_fun
Wake control thread
complete(&st->done);
takedown_cpu
kthread_park
set_bit(KTHREAD_SHOULD_PARK
Here cpuhp is looping,
//success case
Generally when issue is not
coming
it schedule out by below :
ht->thread_should_run(td->cpu
scheduler
//failure case
before schedule
loop check
(kthread_should_park()
enter here as PARKED set
wake_up_process(k)
If k has TASK_PARKED, then wake_up_process() which uses TASK_NORMAL will
no-op, because:
TASK_PARKED & TASK_NORMAL == 0
__kthread_parkme
complete(&self->parked);
SETS RUNNING
schedule
But suppose, you do get that store, and we get to schedule with
TASK_RUNNING, then schedule will no-op and we'll go around the loop and
not complete.
See also: lkml.kernel.org/r/20180430111744.GE4082@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Either TASK_RUNNING gets set before we do schedule() and we go around
again, re-set TASK_PARKED, resched the condition and re-call schedule(),
or we schedule() first and ttwu() will not issue the TASK_RUNNING store.
In either case, we'll eventually hit schedule() with TASK_PARKED. Then,
and only then will the complete() happen.
wait_for_completion(&kthread->parked);
The point is, we'll only ever complete ^ that completion when we've
scheduled out the task in TASK_PARKED state. If the task didn't get
parked, no completion.
Thanks for the detailed explanation, yes in all cases unpark will
observe parked state only.
And that is the reason I like this approach above the others. It
guarantees the task really is parked when we ask for it. We don't have
to deal with the task still running and getting migrated to another CPU
nonsense.
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