linux-kernel - Re: [PATCH] freezer,sched: Rewrite core freezer logic

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Message-ID: <20210615154539.GA30333@redhat.com>
Date:   Tue, 15 Jun 2021 17:45:39 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        Will Deacon <will@...nel.org>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] freezer,sched: Rewrite core freezer logic

On 06/14, Peter Zijlstra wrote:
>
> One more thing; if I add additional state bits to preserve
> __TASK_{TRACED,STOPPED}, then I need to figure out at thaw time if we've
> missed a wakeup or not.
>
> Do we have sufficient state for that? If so, don't we then also not have
> sufficient state to tell if a task should've been TRACED/STOPPED in the
> first place?

Not sure I understand you, probably not, but I think the answer is "no" ;)

But this reminds me... can't we implement selective wakeups? So that if a
task T sleeps in state = STOPPED | FROZEN, then ttwu(T, FROZEN) won't wake
it up, it will only clear FROZEN from T->state. Similarly, ttwu(T, STOPPED)
will leave this task with state == FROZEN.

See also

	https://lore.kernel.org/lkml/20131112162136.GA29065@redhat.com/
	https://lore.kernel.org/lkml/20131113170724.GA17739@redhat.com/

just to remind you that we already discussed TASK_FROZEN a little bit almost
10 years ago ;)

What has been shall be, and what has been done is what will be done, and
there is nothing new under the sun.

Oleg.