linux-kernel - Re: [RFC v0 2/9] suspend: Add getter function to report if freezing is active

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Date:	Mon, 7 Sep 2015 10:55:43 +0200
From:	Daniel Wagner <daniel.wagner@...-carit.de>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	<linux-kernel@...r.kernel.org>, Len Brown <len.brown@...el.com>,
	Pavel Machek <pavel@....cz>, <linux-pm@...r.kernel.org>
Subject: Re: [RFC v0 2/9] suspend: Add getter function to report if freezing
 is active

On 09/05/2015 04:11 AM, Rafael J. Wysocki wrote:
> On Friday, September 04, 2015 03:34:55 PM Daniel Wagner wrote:
>> Instead encode the FREEZE state via the CPU state we allow the
>> interesting subsystems (MCE, microcode) to query the power
>> subsystem directly.
> 
> A use case, please.

The motivation for this change is to reduce the complexity in the
hotplug code. As tried to point out in the cover letter, the FROZEN
bits have only a bunch of users after all those years (2007). So it is
worth to have all the notifier users to handle the FROZEN state?

Don't know if that counts as use case.

>> Most notifiers are not interested at all
>> in this information so rather have explicit calls to freeze_active()
>> instead adding complexity to the rest of the users of the CPU
>> notifiers.
> 
> Why does it has anything to do with CPU notifiers?

cpu_{down|up} will call the notifiers with the CPU_TASK_FROZEN bit set
and so most notifiers are doing

	switch (actcion ~CPU_TASK_FROZEN)

to filter it out because they don't need to handle the system wide
ongoing freeze operations.

> We don't offline CPUs for suspend-to-idle.

Sure. As I said the motivation is to reduce the complexity in the
hotplug code.

Thanks,
Daniel
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