Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)

From: Pavel Machek <pavel@ucw.cz>
To: Ingo Molnar <mingo@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Chanho Min <chanho.min@lge.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)
Date: Tue, 4 Dec 2018 20:29:26 +0100	[thread overview]
Message-ID: <20181204192926.GA28651@amd> (raw)
In-Reply-To: <20181204090228.GC73770@gmail.com>

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Hi!

> * Michal Hocko <mhocko@kernel.org> wrote:
> 
> > > Do we actually have reports of this happening for people outside
> > > Android?
> > 
> > Not that I am aware of.
> 
> I'd say outside of Android 99% of the use of hibernation is the fail-safe 
> that distributions offer on laptops with very low battery levels: the 
> emergency hibernation when there's almost no power left anymore.

Android does not use hibernation AFAICT. Just s2ram.

> Do these hibernation failure messages typically make it to persistent 
> logs before the system uses power?

I'd say so. If you have enough energy left for hibernation, you also
have enough energy left to write the logs and sync.

> In practice if that is buggy the kernel won't hibernate and the laptop 
> will run out of power and the user will conclude "ugh, I shouldn't have 
> left my laptop turned on" - without looking into the logs and reporting, 
> as they'll perceive it as a user failure not a system failure.
> 
> I certainly saw random Linux laptops fail to hibernate over the years and 
> didn't report it, so if the distribution doesn't do the reporting 
> automatically then chances are we'll never see it.

There are many reasons while hibernation can fail. Buggy drivers,
tasks in D state... And there are some when hibernation can fail "by
design". If you swap does not have enough space to store the data, for
example.

Hibernation was designed to be non-intrusive, and reliable as in "if
it hibernates it will also resume ok", but not reliable as in "it will
always hibernate".

I see that is problematic for "hibernate on battery low".

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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