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From: Uladzislau Rezki <>
To: Sebastian Andrzej Siewior <>,
	"Paul E. McKenney" <>,
	Thomas Gleixner <>
Cc: "Paul E. McKenney" <>,
	Thomas Gleixner <>,
	"tip-bot2 for Paul E. McKenney" <>,,
	Uladzislau Rezki <>,
	Peter Zijlstra <>,
	Frederic Weisbecker <>,,
Subject: Re: [tip: core/rcu] softirq: Don't try waking ksoftirqd before it has been spawned
Date: Wed, 14 Apr 2021 10:57:57 +0200	[thread overview]
Message-ID: <20210414085757.GA1917@pc638.lan> (raw)
In-Reply-To: <>

On Wed, Apr 14, 2021 at 09:13:22AM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-04-12 11:36:45 [-0700], Paul E. McKenney wrote:
> > > Color me confused. I did not follow the discussion around this
> > > completely, but wasn't it agreed on that this rcu torture muck can wait
> > > until the threads are brought up?
> > 
> > Yes, we can cause rcutorture to wait.  But in this case, rcutorture
> > is just the messenger, and making it wait would simply be ignoring
> > the message.  The message is that someone could invoke any number of
> > things that wait on a softirq handler's invocation during the interval
> > before ksoftirqd has been spawned.
> My memory on this is that the only user, that required this early
> behaviour, was kprobe which was recently changed to not need it anymore.
> Which makes the test as the only user that remains. Therefore I thought
> that this test will be moved to later position (when ksoftirqd is up and
> running) and that there is no more requirement for RCU to be completely
> up that early in the boot process.
> Did I miss anything?
Seems not. Let me wrap it up a bit though i may miss something:

1) Initially we had an issue with booting RISV because of:

36dadef23fcc ("kprobes: Init kprobes in early_initcall")

i.e. a developer decided to move initialization of kprobe at
early_initcall() phase. Since kprobe uses synchronize_rcu_tasks()
a system did not boot due to the fact that RCU-tasks were setup
at core_initcall() step. It happens later in this chain.

To address that issue, we had decided to move RCU-tasks setup
to before early_initcall() and it worked well:

2) After that fix you reported another issue. If the kernel is run
with "threadirqs=1" - it did not boot also. Because ksoftirqd does
not exist by that time, thus our early-rcu-self test did not pass.

3) Due to (2), Masami Hiramatsu proposed to fix kprobes by delaying
kprobe optimization and it also addressed initial issue:

At the same time Paul made another patch:

softirq: Don't try waking ksoftirqd before it has been spawned

it allows us to keep RCU-tasks initialization before even
early_initcall() where it is now and let our rcu-self-test
to be completed without any hanging.

Vlad Rezki

  reply	other threads:[~2021-04-14  9:00 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-11 13:43 [tip: core/rcu] softirq: Don't try waking ksoftirqd before it has been spawned tip-bot2 for Paul E. McKenney
2021-04-12 14:16 ` Thomas Gleixner
2021-04-12 18:36   ` Paul E. McKenney
2021-04-14  7:13     ` Sebastian Andrzej Siewior
2021-04-14  8:57       ` Uladzislau Rezki [this message]
2021-04-14 18:11         ` Paul E. McKenney
2021-04-14 23:54           ` Thomas Gleixner
2021-04-15  5:02             ` Paul E. McKenney
2021-04-15 14:34               ` Uladzislau Rezki

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