linux-kernel - Re: [PATCH 2/2] rcu-tasks: add RCU-tasks self tests

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Date:   Thu, 18 Feb 2021 14:03:07 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     paulmck@...nel.org
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Michal Hocko <mhocko@...e.com>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        rostedt@...dmis.org
Subject: Re: [PATCH 2/2] rcu-tasks: add RCU-tasks self tests

On Wed, 17 Feb 2021 10:17:38 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > > 1.	Spawn ksoftirqd earlier.
> > > 
> > > 2.	Suppress attempts to awaken ksoftirqd before it exists,
> > > 	forcing all ksoftirq execution on the back of interrupts.
> > > 
> > > Uladzislau and I each produced patches for #1, and I produced a patch
> > > for #2.
> > > 
> > > The only other option I know of is to push the call to init_kprobes()
> > > later in the boot sequence, perhaps to its original subsys_initcall(),
> > > or maybe only as late as core_initcall().  I added Masami and Steve on
> > > CC for their thoughts on this.
> > > 
> > > Is there some other proper fix that I am missing?
> > 
> > Oh, I missed that the synchronize_rcu_tasks() will be involved the kprobes
> > in early stage. Does the problem only exist in the synchronize_rcu_tasks()
> > instead of synchronize_rcu()? If so I can just stop optimizer in early stage
> > because I just want to enable kprobes in early stage, but not optprobes.
> > 
> > Does the following patch help?
> 
> It does look to me like it would!  I clearly should have asked you about
> this a couple of months ago.  ;-)
> 
> The proof of the pudding would be whether the powerpc guys can apply
> this to v5.10-rc7 and have their kernel come up without hanging at boot.

Who could I ask for testing this patch, Uladzislau?
I think the test machine which enough slow or the kernel has much initcall
to run optimization thread while booting.
In my environment, I could not reproduce that issue because the optimizer
was sheduled after some tick passed. At that point, ksoftirqd has already
been initialized.

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>