Re: suspend race -mm regression [Was: Power: fix suspend vt regression]

From: Jiri Slaby
Date: Fri Sep 04 2009 - 07:49:50 EST

On 08/31/2009 09:32 PM, Rafael J. Wysocki wrote:
> On Monday 31 August 2009, Jiri Slaby wrote:
>> On 08/11/2009 11:19 PM, Jiri Slaby wrote:
>>> However there is still a race or something. Sometimes the suspend goes
>>> through, sometimes it doesn't. I will investigate this further.
>> Hmm, this took a loong time to track down a bit. Code instrumentation by
>> outb(XX, 0x80) usually caused the issue to disappear.
>> However I found out that it's caused by might_sleep() calls in
>> flush_workqueue() and flush_cpu_workqueue(). I.e. it looks like there is
>> a task which deadlocks/spins forever. If we won't reschedule to it,
>> suspend proceeds.
>> I replaced the latter might_sleep() by show_state() and removed
>> refrigerated tasks afterwards. The thing is that I don't know if the
>> prank task is there. I need a scheduler to store "next" task pid or
>> whatever to see what it picked as "next" and so what will run due to
>> might_sched(). I can then show it on port 80 display and read it when
>> the hangup occurs.
>> Depending on which might_sleep(), either flush_workqueue() never (well,
>> at least in next 5 minutes) proceeds to for_each_cpu() or
>> wait_for_completion() in flush_cpu_workqueue() never returns.
>> It's a regression against some -rc1 based -next tree. Bisection
>> impossible, suspend needs to be run even 7 times before it occurs. Maybe
>> a s/might_sleep/yield/ could make it happen earlier (going to try)?
> If /sys/class/rtc/rtc0/wakealarm works on this box, you can use it to trigger
> resume in a loop.
> Basically, you can do
> # echo 0 > /sys/class/rtc/rtc0/wakealarm
> # date +%s -d "+60 seconds" > /sys/class/rtc/rtc0/wakealarm
> then go to suspend and it will resume the box in ~1 minute.

Thanks, in the end I found it manually. Goddammit! It's an -mm thing:

Well, I don't know why, but when the kthread overthere runs under
suspend conditions and gets rescheduled (e.g. by the might_sleep()
inside) it never returns. pick_next_task always returns the idle task
from the idle queue. State of the thread is TASK_RUNNING.

Why is it not enqueued into some queue? I tried also
sched_setscheduler(current, FIFO, 99) in the thread itself. Unless I did
it wrong, it seems like a global scheduler problem?

Ingo, any ideas?

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