From: "Paul E. McKenney" <paulmck@kernel.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Masami Hiramatsu <mhiramat@kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Andrew Morton <akpm@linux-foundation.org>,
Daniel Axtens <dja@axtens.net>,
Frederic Weisbecker <frederic@kernel.org>,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Joel Fernandes <joel@joelfernandes.org>,
Michal Hocko <mhocko@suse.com>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization
Date: Mon, 22 Feb 2021 10:16:08 -0800 [thread overview]
Message-ID: <20210222181608.GK2743@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20210222171605.GA42169@pc638.lan>
On Mon, Feb 22, 2021 at 06:16:05PM +0100, Uladzislau Rezki wrote:
> On Mon, Feb 22, 2021 at 07:09:03AM -0800, Paul E. McKenney wrote:
> > On Mon, Feb 22, 2021 at 01:54:31PM +0100, Uladzislau Rezki wrote:
> > > On Mon, Feb 22, 2021 at 11:21:04AM +0100, Sebastian Andrzej Siewior wrote:
> > > > On 2021-02-19 10:33:36 [-0800], Paul E. McKenney wrote:
> > > > > For definiteness, here is the first part of the change, posted earlier.
> > > > > The commit log needs to be updated. I will post the change that keeps
> > > > > the tick going as a reply to this email.
> > > > …
> > > > > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > > > > index 9d71046..ba78e63 100644
> > > > > --- a/kernel/softirq.c
> > > > > +++ b/kernel/softirq.c
> > > > > @@ -209,7 +209,7 @@ static inline void invoke_softirq(void)
> > > > > if (ksoftirqd_running(local_softirq_pending()))
> > > > > return;
> > > > >
> > > > > - if (!force_irqthreads) {
> > > > > + if (!force_irqthreads || !__this_cpu_read(ksoftirqd)) {
> > > > > #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
> > > > > /*
> > > > > * We can safely execute softirq on the current stack if
> > > > > @@ -358,8 +358,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
> > > > >
> > > > > pending = local_softirq_pending();
> > > > > if (pending) {
> > > > > - if (time_before(jiffies, end) && !need_resched() &&
> > > > > - --max_restart)
> > > > > + if (!__this_cpu_read(ksoftirqd) ||
> > > > > + (time_before(jiffies, end) && !need_resched() && --max_restart))
> > > > > goto restart;
> > > >
> > > > This is hunk shouldn't be needed. The reason for it is probably that the
> > > > following wakeup_softirqd() would avoid further invoke_softirq()
> > > > performing the actual softirq work. It would leave early due to
> > > > ksoftirqd_running(). Unless I'm wrong, any raise_softirq() invocation
> > > > outside of an interrupt would do the same.
> >
> > And it does pass the rcutorture test without that hunk:
> >
> > tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 2 --configs "TREE03" --kconfig "CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y" --bootargs "threadirqs=1" --trust-make
> >
> Yep. I have tested that patch also. It works for me as well. So
> technically i do not see any issues from the first glance but of
> course it should be reviewed by the softirq people to hear their
> opinion.
>
> IRQs are enabled, so it can be handled from an IRQ tail until
> ksoftirqd threads are spawned.
And if I add "CONFIG_NO_HZ_IDLE=y CONFIG_HZ_PERIODIC=n" it still works,
even if I revert my changes to rcu_needs_cpu(). Should I rely on this
working globally? ;-)
Thanx, Paul
> > > > I would like PeterZ / tglx to comment on this one. Basically I'm not
> > > > sure if it is okay to expect softirqs beeing served and waited on that
> > > > early in the boot.
> >
> > It would be good to get other eyes on this.
> >
> > I do agree that "don't wait on softirq handlers until after completion
> > of all early_initcall() handlers" is a nice simple rule, but debugging
> > violations of it is not so simple. Adding warnings to ease debugging
> > of violations of this rule is quite a bit more complex than is either of
> > the methods of making the rule unnecessary, at least from what I can see
> > at this point. The complexity of the warnings is exactly what Sebastian
> > pointed out earlier, that it is currently legal to raise_softirq() but
> > not to wait on the resulting handlers. But even waiting is OK if that
> > waiting does not delay the boot sequence. But if the boot kthread waits
> > on the kthread that does the waiting, it is once again not OK.
> >
> > So am I missing something subtle here?
> >
> I agree here. Seems like we are on the same page in understanding :)
>
> > > The ksoftirqd threads get spawned during early_initcall() phase. Why not
> > > just spawn them one step earlier what is totally safe? I mean before
> > > do_pre_smp_initcalls() that calls early callbacks.
> > >
> > > + spawn_ksoftirqd();
> > > rcu_init_tasks_generic();
> > > do_pre_smp_initcalls();
> > >
> > > With such change the spawning will not be depended on linker/compiler
> > > i.e. when and in which order an early_initcall(spawn_ksoftirqd) callback
> > > is executed.
> >
> > We both posted patches similar to this, so I am not opposed. One caveat,
> > though, namely that this narrows the window quite a bit but does not
> > entirely close it. But it does allow the early_initcall()s to wait on
> > softirq handlers.
> >
> Yep, that was an intention. At least to provide such functionality for early
> callbacks. What happens before it(init/main.c) is pretty controllable.
>
> --
> Vlad Rezki
next prev parent reply other threads:[~2021-02-22 18:44 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-18 14:29 [PATCH] kprobes: Fix to delay the kprobes jump optimization Masami Hiramatsu
2021-02-18 15:15 ` Paul E. McKenney
2021-02-19 8:17 ` Sebastian Andrzej Siewior
2021-02-19 10:49 ` Uladzislau Rezki
2021-02-19 10:57 ` Sebastian Andrzej Siewior
2021-02-19 11:13 ` Uladzislau Rezki
2021-02-19 11:17 ` Sebastian Andrzej Siewior
2021-02-19 11:23 ` Uladzislau Rezki
2021-02-19 11:27 ` Uladzislau Rezki
2021-02-19 18:18 ` Paul E. McKenney
2021-02-19 18:33 ` Paul E. McKenney
2021-02-19 19:34 ` Paul E. McKenney
2021-02-19 20:02 ` Steven Rostedt
2021-02-19 21:22 ` Paul E. McKenney
2021-02-22 10:21 ` Sebastian Andrzej Siewior
2021-02-22 12:54 ` Uladzislau Rezki
2021-02-22 15:09 ` Paul E. McKenney
2021-02-22 17:16 ` Uladzislau Rezki
2021-02-22 18:16 ` Paul E. McKenney [this message]
2021-02-22 19:07 ` Uladzislau Rezki
2021-02-22 21:32 ` Paul E. McKenney
2021-02-19 19:14 ` Steven Rostedt
2021-02-19 19:45 ` Paul E. McKenney
2021-02-19 20:04 ` Paul E. McKenney
2021-02-22 10:04 ` Sebastian Andrzej Siewior
2021-02-19 19:36 ` Steven Rostedt
2021-02-19 19:47 ` Paul E. McKenney
2021-02-19 19:58 ` Steven Rostedt
2021-02-19 20:04 ` Paul E. McKenney
2021-02-22 12:06 ` Masami Hiramatsu
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