linux-kernel - Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization

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Date:   Mon, 22 Feb 2021 13:54:31 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>
Subject: Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization

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. 
> 
> 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.
> 
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.

--
Vlad Rezki