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

[Paul E. McKenney]

On Fri, Feb 12, 2021 at 04:37:09PM -0800, Paul E. McKenney wrote:
> On Fri, Feb 12, 2021 at 03:48:51PM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 12, 2021 at 10:12:07PM +0100, Uladzislau Rezki wrote:
> > > On Fri, Feb 12, 2021 at 08:20:59PM +0100, Sebastian Andrzej Siewior wrote:
> > > > On 2020-12-09 21:27:32 [+0100], Uladzislau Rezki (Sony) wrote:
> > > > > Add self tests for checking of RCU-tasks API functionality.
> > > > > It covers:
> > > > >     - wait API functions;
> > > > >     - invoking/completion call_rcu_tasks*().
> > > > > 
> > > > > Self-tests are run when CONFIG_PROVE_RCU kernel parameter is set.
> > > > 
> > > > I just bisected to this commit. By booting with `threadirqs' I end up
> > > > with:
> > > > [    0.176533] Running RCU-tasks wait API self tests
> > > > 
> > > > No stall warning or so.
> > > > It boots again with:
> > > > 
> > > > diff --git a/init/main.c b/init/main.c
> > > > --- a/init/main.c
> > > > +++ b/init/main.c
> > > > @@ -1489,6 +1489,7 @@ void __init console_on_rootfs(void)
> > > >  	fput(file);
> > > >  }
> > > >  
> > > > +void rcu_tasks_initiate_self_tests(void);
> > > >  static noinline void __init kernel_init_freeable(void)
> > > >  {
> > > >  	/*
> > > > @@ -1514,6 +1515,7 @@ static noinline void __init kernel_init_freeable(void)
> > > >  
> > > >  	rcu_init_tasks_generic();
> > > >  	do_pre_smp_initcalls();
> > > > +	rcu_tasks_initiate_self_tests();
> > > >  	lockup_detector_init();
> > > >  
> > > >  	smp_init();
> > > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > > --- a/kernel/rcu/tasks.h
> > > > +++ b/kernel/rcu/tasks.h
> > > > @@ -1266,7 +1266,7 @@ static void test_rcu_tasks_callback(struct rcu_head *rhp)
> > > >  	rttd->notrun = true;
> > > >  }
> > > >  
> > > > -static void rcu_tasks_initiate_self_tests(void)
> > > > +void rcu_tasks_initiate_self_tests(void)
> > > >  {
> > > >  	pr_info("Running RCU-tasks wait API self tests\n");
> > > >  #ifdef CONFIG_TASKS_RCU
> > > > @@ -1322,7 +1322,6 @@ void __init rcu_init_tasks_generic(void)
> > > >  #endif
> > > >  
> > > >  	// Run the self-tests.
> > > > -	rcu_tasks_initiate_self_tests();
> > > >  }
> > > >  
> > > >  #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
> > > > 
> > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > 
> > Apologies for the hassle!  My testing clearly missed this combination
> > of CONFIG_PROVE_RCU=y and threadirqs=1.  :-(
> > 
> > But at least I can easily reproduce this hang as follows:
> > 
> > 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
> > 
> > Sadly, I cannot take your patch because that simply papers over the
> > fact that early boot use of synchronize_rcu_tasks() is broken in this
> > particular configuration, which will likely eventually bite others now
> > that init_kprobes() has been moved earlier in boot:
> > 
> > 1b04fa990026 ("rcu-tasks: Move RCU-tasks initialization to before early_initcall()")
> > Link: https://lore.kernel.org/rcu/87eekfh80a.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> > Fixes: 36dadef23fcc ("kprobes: Init kprobes in early_initcall")
> > 
> > > > Sebastian
> > > >
> > > We should be able to use call_rcu_tasks() in the *initcall() callbacks.
> > > The problem is that, ksoftirqd threads are not spawned by the time when
> > > an rcu_init_tasks_generic() is invoked:
> > > 
> > > diff --git a/init/main.c b/init/main.c
> > > index c68d784376ca..e6106bb12b2d 100644
> > > --- a/init/main.c
> > > +++ b/init/main.c
> > > @@ -954,7 +954,6 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
> > >  	rcu_init_nohz();
> > >  	init_timers();
> > >  	hrtimers_init();
> > > -	softirq_init();
> > >  	timekeeping_init();
> > >  
> > >  	/*
> > > @@ -1512,6 +1511,7 @@ static noinline void __init kernel_init_freeable(void)
> > >  
> > >  	init_mm_internals();
> > >  
> > > +	softirq_init();
> > >  	rcu_init_tasks_generic();
> > >  	do_pre_smp_initcalls();
> > >  	lockup_detector_init();
> > > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > > index 9d71046ea247..cafa55c496d0 100644
> > > --- a/kernel/softirq.c
> > > +++ b/kernel/softirq.c
> > > @@ -630,6 +630,7 @@ void __init softirq_init(void)
> > >  			&per_cpu(tasklet_hi_vec, cpu).head;
> > >  	}
> > >  
> > > +	spawn_ksoftirqd();
> > 
> > We need a forward reference to allow this to build, but with that added,
> > my test case passes.  Good show!
> > 
> > >  	open_softirq(TASKLET_SOFTIRQ, tasklet_action);
> > >  	open_softirq(HI_SOFTIRQ, tasklet_hi_action);
> > >  }
> > > @@ -732,7 +733,6 @@ static __init int spawn_ksoftirqd(void)
> > >  
> > >  	return 0;
> > >  }
> > > -early_initcall(spawn_ksoftirqd);
> > >  
> > >  /*
> > >   * [ These __weak aliases are kept in a separate compilation unit, so that
> > > 
> > > Any thoughts?
> > 
> > One likely problem is that there are almost certainly parts of the kernel
> > that need softirq_init() to stay roughly where it is.  So, is it possible
> > to leave softirq_init() where it is, and to arrange for spawn_ksoftirqd()
> > to be invoked just before rcu_init_tasks_generic() is called?
> 
> This still seems worth trying (and doing so is next on my list), but just

And the patch below takes this approach, which also causes the tests to
pass.

Thoughts?

								Thanx, Paul

------------------------------------------------------------------------

commit f4cd768e341486655c8c196e1f2b48a4463541f3
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date:   Fri Feb 12 16:41:05 2021 -0800

    softirq: Don't try waking ksoftirqd before it has been spawned
    
    If there is heavy softirq activity, the softirq system will attempt
    to awaken ksoftirqd and will stop the traditional back-of-interrupt
    softirq processing.  This is all well and good, but only if the
    ksoftirqd kthreads already exist, which is not the case during early
    boot, in which case the system hangs.
    
    One reproducer is as follows:
    
    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
    
    This commit therefore moves the spawning of the ksoftirqd kthreads
    earlier in boot.  With this change, the above test passes.
    
    Reported-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
    Reported-by: Uladzislau Rezki <urezki@xxxxxxxxx>
    Inspired-by: Uladzislau Rezki <urezki@xxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index bb8ff90..283a02d 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -592,6 +592,8 @@ static inline struct task_struct *this_cpu_ksoftirqd(void)
 	return this_cpu_read(ksoftirqd);
 }
 
+int spawn_ksoftirqd(void);
+
 /* Tasklets --- multithreaded analogue of BHs.
 
    This API is deprecated. Please consider using threaded IRQs instead:
diff --git a/init/main.c b/init/main.c
index c68d784..99835bb 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1512,6 +1512,7 @@ static noinline void __init kernel_init_freeable(void)
 
 	init_mm_internals();
 
+	spawn_ksoftirqd();
 	rcu_init_tasks_generic();
 	do_pre_smp_initcalls();
 	lockup_detector_init();
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 9d71046..45d50d4 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -724,7 +724,7 @@ static struct smp_hotplug_thread softirq_threads = {
 	.thread_comm		= "ksoftirqd/%u",
 };
 
-static __init int spawn_ksoftirqd(void)
+__init int spawn_ksoftirqd(void)
 {
 	cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
 				  takeover_tasklets);
@@ -732,7 +732,6 @@ static __init int spawn_ksoftirqd(void)
 
 	return 0;
 }
-early_initcall(spawn_ksoftirqd);
 
 /*
  * [ These __weak aliases are kept in a separate compilation unit, so that