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

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Message-ID: <13709135.uLZWGnKmhe@rjwysocki.net>
Date: Tue, 25 Feb 2025 17:38:01 +0100
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Alan Stern <stern@...land.harvard.edu>, Ulf Hansson <ulf.hansson@...aro.org>,
 Johan Hovold <johan@...nel.org>,
 Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
 Saravana Kannan <saravanak@...gle.com>
Subject:
 [PATCH v1 0/5] PM: sleep: Improvements of async suspend and resume of devices

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@google.com/

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.

The first two patches in the series are preparatory.

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.

Thanks!