Re: [PATCH v1 0/5] PM: sleep: Improvements of async suspend and resume of devices

[Ulf Hansson]

On Tue, 25 Feb 2025 at 17:46, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
>
> Hi Everyone,
>
> Initially, this was an attempt to address the problems described by
> Saravana related to spawning async work for any async device upfront
> in the resume path:
>
> https://lore.kernel.org/linux-pm/20241114220921.2529905-1-saravanak@xxxxxxxxxx/
>
> but then I realized that it could be extended to the suspend path and
> used for speeding it up, which it really does.
>
> Overall, the idea is that instead of starting an async work item for every
> async device upfront, which is not very efficient because the majority of
> those devices will not be able to make progress due to dependencies anyway,
> the async handling is only started upfront for the devices that are likely
> to be able to make progress.  That is, devices without parents in the resume
> path and leaf devices (ie. devices without children or consumers) in the
> suspend path (the underlying observation here is that devices without parents
> are likely to have no suppliers too whereas devices without children that
> have consumers are not unheard of).  This allows to reduce the amount of
> processing that needs to be done to start with.
>
> Then, after processing every device ("async" or "sync"), "async" processing
> is started for some devices that have been "unblocked" by it, which are its
> children in the resume path or its parent and its suppliers in the suspend
> path.  This allows asynchronous handling to start as soon as it makes sense
> without delaying the "async" devices unnecessarily.
>
> Fortunately, the additional plumbing needed to implement this is not
> particularly complicated.

Thanks for the detailed description! Overall, the approach makes
perfect sense to me too!

I am certainly interested to hear Saravana's thoughts around this too.

>
> The first two patches in the series are preparatory.

For these two, feel free to add:

Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>

>
> Patch [3/5] deals with the resume path for all device resume phases.
>
> Patch [4/5] optimizes the "suspend" phase which has the most visible effect (on
> the systems in my office the speedup is in the 100 ms range which is around 20%
> of the total device resume time).
>
> Patch [5/5] extend this to the "suspend late" and "suspend noirq" phases.

I will try to have a closer look at patch 3->5 later in the week.

Kind regards
Uffe