Re: [PATCH] PM: s2idle: Introduce syscore callbacks in s2idle flow

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Thursday, September 3, 2020 4:14:07 AM CEST Claude. Yen wrote:
> On Tue, 2020-09-01 at 13:57 +0200, Rafael J. Wysocki wrote:
> > On Tue, Sep 1, 2020 at 9:05 AM Claude Yen <claude.yen@mediatek.com> wrote:
> > >
> > > This series based on 5.9-rc1
> > > This patch makes s2idle call existing syscore callbacks. Currently,
> > > when s2idle is selected as system suspend method, callbacks hooked
> > > by register_syscore_ops() will not be triggered. This may induce
> > > unexpected results.
> > 
> > They are not executed by design.
> > 
> > > For example, sched_clock_suspend() was added to s2idle flow in
> > > commit 3f2552f7e9c5 ("timers/sched_clock: Prevent generic sched_clock
> > > wrap caused by tick_freeze()") to fix clock wrap problem. However,
> > > sched_clock_suspend() is originally registered in syscore callback.
> > 
> > I'm not sure why this matters here.
> 
> If functions in syscore callbacks are needed in s2idle, explicit
> migration is needed like commit 3f2552f7e9c5 ("timers/sched_clock:
> Prevent generic sched_clock wrap caused by tick_freeze()").
> Thus, I am wondering if such effort could be saved.

Yes, it could.

You can define platform ops for s2idle and invoke what's needed from there.

> > > With this patch, if another syscore callback is needed in s2idle,
> > > additional migration effort could be saved.
> > 
> > s2idle cannot execute syscore callbacks, because it doesn' take
> > non-boot CPUs offline and it won't do that.
> > 
> > Thanks!
> 
> Yes, the current design of syscore callback needs non-boot CPUs offline.
> Considering the following case: in s2idle flow, there is a status that
> only one CPU is alive and other CPUs have enter deepest idle state.
> This situation is similar to getting non-boot CPUs offline, though all
> CPUs are online from kernel's perspective.

It is only similar AFAICS.

You don't migrate interrupts during s2idle, for example.

> Reply from Stephen mentioned that if an operation is needed in both
> S2R and s2idle, CPU_PM notifier can be utilized. 
> In my opinion, CPU_PM notifier is particularly for CPU entering idle
> state. In contrast, syscore callback is for system going low power
> state. There exists semantic difference between these two callbacks.

Fair enough.

> Could the current design of syscore callback be re-designed as
> system-wide suspend callback?

No, it couldn't.

> Proposed suspend flow in this patch:
> 
>     Freeze tasks
>         |
>         V
>     Device suspend callbacks
>         |
>         |-------------s2idle----------
>         |                            |
>         V                            |
>     Disable nonboot CPUs    Is this CPU last core to enter idle?
>         |                            |
>         V                            |-------------
>     syscore callbacks                |            |
>         |                           No           Yes
>         V                            |            |
>     platform suspend                 V            V
>                                  enter idle     syscore callback
>                                                   |
>                                                   V
>                                                 enter idle
> 

The primary problem with this is that on some architectures (x86 at least)
the syscore things cannot be run during the s2idle flow.

Also there is a way to invoke them through the platform ops as I said.

Thanks!