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

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Date:   Tue, 4 Dec 2018 10:58:02 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Pavel Machek <pavel@....cz>, Oleg Nesterov <oleg@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Chanho Min <chanho.min@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux
 4.20-rc4)

On Tue 04-12-18 10:33:10, Ingo Molnar wrote:
> 
> * Michal Hocko <mhocko@...nel.org> wrote:
> 
> > I dunno. I do not use hibernation. I am a heavy user of the suspend 
> > though. I s2ram all the time. And I have certainly experienced cases 
> > where suspend has failed and I onlyi found out later when I've picked 
> > up my laptop from my heat up bag. Nothing fatal has resulted from that 
> > but this is certainly annoying.
> 
> Hm, so I (mistakenly) thought freezing was mostly limited to hibernation 
> and to a few weird cases when in flight DMA must not be suspended - but 
> I'm wrong and in practice we always freeze tasks during s2ram, right?

Yup.

> And indeed:
> 
>  config SUSPEND_FREEZER
>         bool "Enable freezer for suspend to RAM/standby" \
>                 if ARCH_WANTS_FREEZER_CONTROL || BROKEN
>         depends on SUSPEND
>         default y
> 
> which is essentially always enabled on x86.
> 
> TIL ...
> 
> s2ram is obviously a huge deal.
> 
> Just a newbie question: any chance to not do any freezing at all on 
> modern laptops when doing s2ram, or at least only warn if it fails and 
> try to suspend?

AFAIU both suspend and hibernation require the system to enter quiescent
state with no task potentially interfering with suspended devices. And
in this particular case those de-thread-ed threads will certainly not
interfere so silencing the lockdep sounds like a reasonable workaround.
-- 
Michal Hocko
SUSE Labs