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

[Paul E. McKenney]

On Mon, Feb 22, 2021 at 08:07:03PM +0100, Uladzislau Rezki wrote:
> On Mon, Feb 22, 2021 at 10:16:08AM -0800, Paul E. McKenney wrote:
> > 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?  ;-)
> > 
> There might be corner cases which we are not aware of so far. From the
> other hand what the patch does is simulating the !threadirqs behaviour
> during early boot. In that case we know that handling of SW irqs from
> real-irq tail works :)

Sold!  I keep the rcu_needs_cpu() changes, just in case.

							Thanx, Paul