Re: [PATCH] tracing: Add trace_trigger kernel command line option

[Paul E. McKenney]

On Thu, Oct 20, 2022 at 12:33:57PM -0400, Steven Rostedt wrote:
> On Thu, 20 Oct 2022 08:59:00 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> 
> > On Wed, Oct 19, 2022 at 08:07:45PM -0400, Steven Rostedt wrote:
> > > On Wed, 19 Oct 2022 20:01:37 -0400
> > > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > >   
> > > > @@ -90,6 +90,10 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
> > > >  #ifdef CONFIG_TRACEPOINTS
> > > >  static inline void tracepoint_synchronize_unregister(void)
> > > >  {
> > > > +	/* Early updates do not need synchronization */
> > > > +	if (early_boot_irqs_disabled)
> > > > +		return;
> > > > +
> > > >  	synchronize_srcu(&tracepoint_srcu);
> > > >  	synchronize_rcu();  
> > > 
> > > I wonder if this check should be just moved to the RCU synchronization
> > > code? That is, if early_boot_irqs_disabled is set, do nothing, as there
> > > should be nothing to synchronize against.  
> > 
> > There already is a similar check, but it follows the lockdep check.
> > 
> > Does the following patch help?
> 
> Not sure if it would.
> 
> I added this:
> 
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c03fd7037add..79ac31a6a87b 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2748,9 +2748,12 @@ void trace_buffered_event_disable(void)
>  			       disable_trace_buffered_event, NULL, 1);
>  	preempt_enable();
>  
> +	if (!irqs_disabled())
> +		printk("IRQS DISABLED!! before %s:%d\n", __func__, __LINE__);
>  	/* Wait for all current users to finish */
> -	if (!early_boot_irqs_disabled)
> -		synchronize_rcu();
> +	synchronize_rcu();
> +	if (!irqs_disabled())
> +		printk("IRQS DISABLED!! after %s:%d\n", __func__, __LINE__);
>  
>  	for_each_tracing_cpu(cpu) {
>  		free_page((unsigned long)per_cpu(trace_buffered_event, cpu));
> 
> Which produced this:
> 
> 
> [    0.972867] ftrace: allocating 47021 entries in 184 pages
> [    0.978285] ftrace section at ffffffffacef74c0 sorted properly
> [    0.991153] ftrace: allocated 184 pages with 4 groups
> [    0.996163] Dynamic Preempt: full
> [    0.999354] rcu: Preemptible hierarchical RCU implementation.
> [    1.005020] rcu:     RCU restricting CPUs from NR_CPUS=128 to nr_cpu_ids=8.
> [    1.011680]  Tasks-RCU CPU stall warnings timeout set to 120000 (rcu_task_stall_timeout).
> [    1.019813]  Trampoline variant of Tasks RCU enabled.
> [    1.024831]  Rude variant of Tasks RCU enabled.
> [    1.029330]  Tracing variant of Tasks RCU enabled.
> [    1.034089] rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
> [    1.041699] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=8
> [    1.052607] IRQS DISABLED!! after trace_buffered_event_disable:2756
> [    1.058712] NR_IRQS: 8448, nr_irqs: 488, preallocated irqs: 16
> [    1.064670] NO_HZ: Full dynticks CPUs: 3.
> [    1.068501] rcu:     Offload RCU callbacks from CPUs: 3.
> [    1.073519] rcu: srcu_init: Setting srcu_struct sizes based on contention.
> [    1.080400] ------------[ cut here ]------------
> [    1.084938] Interrupts were enabled early
> [    1.088919] WARNING: CPU: 0 PID: 0 at init/main.c:1061 start_kernel+0x36f/0x4e8
> [    1.096184] Modules linked in:
> [    1.099213] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.1.0-rc1-test+ #468
> [    1.106047] Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
> [    1.114960] RIP: 0010:start_kernel+0x36f/0x4e8
> [    1.119373] Code: 48 27 53 e8 fd 7b 03 00 e8 55 72 e5 fe e8 6b 05 03 00 ff 15 2a f9 89 ff 0f ba e0 09 73 0e 48 c7 c7 98 25 31 ac e8 01 5b df fe <0f> 0b c6 05 c7 2c af ff 00 e8 ba 36 47 fe ff 15 14 f9 89 ff e8 56
> [    1.138057] RSP: 0000:ffffffffac603f30 EFLAGS: 00010286
> [    1.143248] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> [    1.150343] RDX: 0000000000000002 RSI: 00000000ffffdfff RDI: 0000000000000001
> [    1.157438] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffffffac603dd8
> [    1.164537] R10: 0000000000000003 R11: ffffffffac734168 R12: 000000000000180a
> [    1.171633] R13: 00000000010fecf0 R14: 0000000000000000 R15: 00000000d44e3018
> [    1.178730] FS:  0000000000000000(0000) GS:ffff93ee1aa00000(0000) knlGS:0000000000000000
> [    1.186778] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    1.192489] CR2: ffff93ee1edff000 CR3: 000000002e60a001 CR4: 00000000000606f0
> [    1.199584] Call Trace:
> [    1.202009]  <TASK>
> 
> 
> So I'm not sure how just updating srcu will do anything to prevent this.

Apologies, SRCU on the brain due to a different email thread...  :-/

							Thanx, Paul

> -- Steve
> 
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index ca4b5dcec675b..356ef70d5442c 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -1267,14 +1267,15 @@ static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
> >  {
> >  	struct rcu_synchronize rcu;
> >  
> > +	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
> > +		return;
> > +
> >  	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
> >  			 lock_is_held(&rcu_bh_lock_map) ||
> >  			 lock_is_held(&rcu_lock_map) ||
> >  			 lock_is_held(&rcu_sched_lock_map),
> >  			 "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
> >  
> > -	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
> > -		return;
> >  	might_sleep();
> >  	check_init_srcu_struct(ssp);
> >  	init_completion(&rcu.completion);
>