Re: [PATCH v1] PM: sleep: Restore asynchronous device resume optimization

Konrad Dybcio
2024-02-10T14:04:19Z

On 2/7/24 11:59, Rafael J. Wysocki wrote:
> On Wed, Feb 7, 2024 at 11:31 AM Marek Szyprowski
> <m.szyprowski@samsung.com> wrote:
>>
>> Dear All,
>>
>> On 09.01.2024 17:59, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>>
>>> Before commit 7839d0078e0d ("PM: sleep: Fix possible deadlocks in core
>>> system-wide PM code"), the resume of devices that were allowed to resume
>>> asynchronously was scheduled before starting the resume of the other
>>> devices, so the former did not have to wait for the latter unless
>>> functional dependencies were present.
>>>
>>> Commit 7839d0078e0d removed that optimization in order to address a
>>> correctness issue, but it can be restored with the help of a new device
>>> power management flag, so do that now.
>>>
>>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>> ---
>>
>> This patch finally landed in linux-next some time ago as 3e999770ac1c
>> ("PM: sleep: Restore asynchronous device resume optimization"). Recently
>> I found that it causes a non-trivial interaction with commit
>> 5797b1c18919 ("workqueue: Implement system-wide nr_active enforcement
>> for unbound workqueues"). Since merge commit 954350a5f8db in linux-next
>> system suspend/resume fails (board doesn't wake up) on my old Samsung
>> Exynos4412-based Odroid-U3 board (ARM 32bit based), which was rock
>> stable for last years.
>>
>> My further investigations confirmed that the mentioned commits are
>> responsible for this issue. Each of them separately (3e999770ac1c and
>> 5797b1c18919) doesn't trigger any problems. Reverting any of them on top
>> of linux-next (with some additional commit due to code dependencies)
>> also fixes/hides the problem.
>>
>> Let me know if You need more information or tests on the hardware. I'm
>> open to help debugging this issue.
>
> So I found this report:
>
> https://lore.kernel.org/lkml/b3d08cd8-d77f-45dd-a2c3-4a4db5a98dfa@kernel.org/
>
> which appears to be about the same issue.
>
> Strictly speaking, the regression is introduced by 5797b1c18919,
> because it is not a mainline commit yet, but the information regarding
> the interaction of it with commit 3e999770ac1c is valuable.
>
> Essentially, what commit 3e999770ac1c does is to schedule the
> execution of all of the async resume callbacks in a given phase
> upfront, so they can run without waiting for the sync ones to complete
> (except when there is a parent-child or supplier-consumer dependency -
> the latter represented by a device link).
>
> What seems to be happening after commit 5797b1c18919 is that one (or
> more) of the async callbacks get blocked in the workqueue for some
> reason.
>
> I would try to replace system_unbound_wq in
> __async_schedule_node_domain() with a dedicated workqueue that is not
> unbound and see what happens.

Thanks Rafael for helping connect the dots!

I did the laziest things imaginable to switch to a WQ that's
not unbound:

diff --git a/kernel/async.c b/kernel/async.c
index 97f224a5257b..37f1204ab4e9 100644
--- a/kernel/async.c
+++ b/kernel/async.c
@@ -174,7 +174,7 @@ static async_cookie_t __async_schedule_node_domain(async_func_t func,
spin_unlock_irqrestore(&async_lock, flags);

/* schedule for execution */
- queue_work_node(node, system_unbound_wq, &entry->work);
+ queue_work_node(node, system_wq, &entry->work);

..and the system can now suspend/resume all day long again!

Konrad