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Date:   Sun, 24 Oct 2021 20:26:14 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Daniel Bristot de Oliveira <bristot@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Zanussi <zanussi@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        John Kacur <jkacur@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-trace-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 04/19] trace/osnoise: Support a list of trace_array *tr

On Fri, Oct 22, 2021 at 10:38:39PM -0400, Steven Rostedt wrote:
> On Thu, 21 Oct 2021 14:56:42 +0200
> Daniel Bristot de Oliveira <bristot@...nel.org> wrote:
> 
> > osnoise/timerlat were built to run a single instance, and for this,
> > a single variable is enough to store the current struct trace_array
> > *tr with information about the tracing instance. This is done via
> > the *osnoise_trace variable. A trace_array represents a trace instance.
> > 
> > In preparation to support multiple instances, replace the
> > *osnoise_trace variable with an RCU protected list of instances.
> > 
> > The operations that refer to an instance now propagate to all
> > elements of the list (all instances).
> > 
> > Also, replace the osnoise_busy variable with a check if the list
> > has elements (busy).
> > 
> > No functional change is expected with this patch, i.e., only one
> > instance is allowed yet.
> > 
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Tom Zanussi <zanussi@...nel.org>
> > Cc: Masami Hiramatsu <mhiramat@...nel.org>
> > Cc: Juri Lelli <juri.lelli@...hat.com>
> > Cc: Clark Williams <williams@...hat.com>
> > Cc: John Kacur <jkacur@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Cc: Daniel Bristot de Oliveira <bristot@...nel.org>
> > Cc: linux-rt-users@...r.kernel.org
> > Cc: linux-trace-devel@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Daniel Bristot de Oliveira <bristot@...nel.org>
> > ---
> >  kernel/trace/trace_osnoise.c | 253 +++++++++++++++++++++++++----------
> >  1 file changed, 184 insertions(+), 69 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
> > index 9b9e6bc889e9..a6d8f514bd7c 100644
> > --- a/kernel/trace/trace_osnoise.c
> > +++ b/kernel/trace/trace_osnoise.c
> > @@ -38,8 +38,6 @@
> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/osnoise.h>
> >  
> > -static struct trace_array	*osnoise_trace;
> > -
> >  /*
> >   * Default values.
> >   */
> > @@ -50,6 +48,73 @@ static struct trace_array	*osnoise_trace;
> >  #define DEFAULT_TIMERLAT_PERIOD	1000			/* 1ms */
> >  #define DEFAULT_TIMERLAT_PRIO	95			/* FIFO 95 */
> >  
> > +/*
> > + * trace_array of the enabled osnoise/timerlat instances.
> > + */
> > +struct osnoise_instance {
> > +	struct list_head	list;
> > +	struct trace_array	*tr;
> > +};
> > +struct list_head osnoise_instances;
> > +
> > +static bool osnoise_has_registered_instances(void)
> > +{
> > +	return !!list_first_or_null_rcu(&osnoise_instances,
> > +					struct osnoise_instance,
> > +					list);
> > +}
> > +
> > +/*
> > + * osnoise_register_instance - register a new trace instance
> > + *
> > + * Register a trace_array *tr in the list of instances running
> > + * osnoise/timerlat tracers.
> > + */
> > +static int osnoise_register_instance(struct trace_array *tr)
> > +{
> > +	struct osnoise_instance *inst;
> > +
> > +	inst = kmalloc(sizeof(*inst), GFP_KERNEL);
> > +	if (!inst)
> > +		return -ENOMEM;
> > +
> > +	INIT_LIST_HEAD_RCU(&inst->list);
> > +	inst->tr = tr;
> > +	list_add_tail_rcu(&inst->list, &osnoise_instances);
> > +
> > +	return 0;
> > +}
> > +
> > +/*
> > + *  osnoise_unregister_instance - unregister a registered trace instance
> > + *
> > + * Remove the trace_array *tr from the list of instances running
> > + * osnoise/timerlat tracers.
> > + */
> > +static void osnoise_unregister_instance(struct trace_array *tr)
> > +{
> > +	struct osnoise_instance *inst;
> > +	int found = 0;
> > +
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(inst, &osnoise_instances, list) {
> > +		if (inst->tr == tr) {
> > +			list_del_rcu(&inst->list);
> 
> Adding to the list requires more than RCU protection, and if this has
> that protection there's no reason for using rcu to protect here.
> 
> [ Added Paul to comment about this too ]

Steve has it right.  To see this, suppose that there are two concurrent
calls to osnoise_unregister_instance() on elements that are adjacent in
the list.  The result will of course be a corrupted list.  All RCU does
is coordinate between readers on the one hand and updaters on the other.
There must be some non-RCU synchronization between updaters, be this a
lock, some lockless synchronization, there being only one task permitted
to carry out updates, or whatever.

So what prevents concurrent calls to osnoise_unregister_instance()?  I am
not seeing anything preventing this is osnoise_tracer_stop().  Is there
some appropriate serialization in the callers of the osnoise_tracer_stop()
function?

							Thanx, Paul

> > +			found = 1;
> > +			break;
> > +		}
> > +	}
> > +	rcu_read_unlock();
> > +
> > +	if (!found) {
> > +		WARN("osnoise unregister did not find tr %s\n", tr->name);
> > +		return;
> > +	}
> > +
> > +	synchronize_rcu();
> > +	kfree(inst);
> > +}
> > +
> >  /*
> >   * NMI runtime info.
> >   */
> > @@ -248,11 +313,6 @@ static struct osnoise_data {
> >  #endif
> >  };
> >  
> > -/*
> > - * Boolean variable used to inform that the tracer is currently sampling.
> > - */
> > -static bool osnoise_busy;
> > -
> >  #ifdef CONFIG_PREEMPT_RT
> >  /*
> >   * Print the osnoise header info.
> > @@ -315,19 +375,24 @@ static void print_osnoise_headers(struct seq_file *s)
> >   * osnoise_taint - report an osnoise error.
> >   */
> >  #define osnoise_taint(msg) ({							\
> > -	struct trace_array *tr = osnoise_trace;					\
> > +	struct osnoise_instance *inst;						\
> > +	struct trace_buffer *buffer;						\
> >  										\
> > -	trace_array_printk_buf(tr->array_buffer.buffer, _THIS_IP_, msg);	\
> > +	rcu_read_lock();							\
> > +	list_for_each_entry_rcu(inst, &osnoise_instances, list) {		\
> > +		buffer = inst->tr->array_buffer.buffer;				\
> > +		trace_array_printk_buf(buffer, _THIS_IP_, msg);			\
> > +	}									\
> > +	rcu_read_unlock();							\
> >  	osnoise_data.tainted = true;						\
> >  })
> >  
> >  /*
> >   * Record an osnoise_sample into the tracer buffer.
> >   */
> > -static void trace_osnoise_sample(struct osnoise_sample *sample)
> > +static void
> > +__trace_osnoise_sample(struct osnoise_sample *sample, struct trace_buffer *buffer)
> >  {
> > -	struct trace_array *tr = osnoise_trace;
> > -	struct trace_buffer *buffer = tr->array_buffer.buffer;
> >  	struct trace_event_call *call = &event_osnoise;
> >  	struct ring_buffer_event *event;
> >  	struct osnoise_entry *entry;
> > @@ -350,6 +415,22 @@ static void trace_osnoise_sample(struct osnoise_sample *sample)
> >  		trace_buffer_unlock_commit_nostack(buffer, event);
> >  }
> >  
> > +/*
> > + * Record an osnoise_sample on all osnoise instances.
> > + */
> > +static void trace_osnoise_sample(struct osnoise_sample *sample)
> > +{
> > +	struct osnoise_instance *inst;
> > +	struct trace_buffer *buffer;
> > +
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(inst, &osnoise_instances, list) {
> > +		buffer = inst->tr->array_buffer.buffer;
> > +		__trace_osnoise_sample(sample, buffer);
> > +	}
> > +	rcu_read_unlock();
> > +}
> > +
> >  #ifdef CONFIG_TIMERLAT_TRACER
> >  /*
> >   * Print the timerlat header info.
> > @@ -387,14 +468,10 @@ static void print_timerlat_headers(struct seq_file *s)
> >  }
> >  #endif /* CONFIG_PREEMPT_RT */
> >  
> > -/*
> > - * Record an timerlat_sample into the tracer buffer.
> > - */
> > -static void trace_timerlat_sample(struct timerlat_sample *sample)
> > +static void
> > +__trace_timerlat_sample(struct timerlat_sample *sample, struct trace_buffer *buffer)
> >  {
> > -	struct trace_array *tr = osnoise_trace;
> >  	struct trace_event_call *call = &event_osnoise;
> > -	struct trace_buffer *buffer = tr->array_buffer.buffer;
> >  	struct ring_buffer_event *event;
> >  	struct timerlat_entry *entry;
> >  
> > @@ -411,6 +488,22 @@ static void trace_timerlat_sample(struct timerlat_sample *sample)
> >  		trace_buffer_unlock_commit_nostack(buffer, event);
> >  }
> >  
> > +/*
> > + * Record an timerlat_sample into the tracer buffer.
> > + */
> > +static void trace_timerlat_sample(struct timerlat_sample *sample)
> > +{
> > +	struct osnoise_instance *inst;
> > +	struct trace_buffer *buffer;
> > +
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(inst, &osnoise_instances, list) {
> > +		buffer = inst->tr->array_buffer.buffer;
> > +		__trace_timerlat_sample(sample, buffer);
> > +	}
> > +	rcu_read_unlock();
> > +}
> > +
> >  #ifdef CONFIG_STACKTRACE
> >  
> >  #define	MAX_CALLS	256
> > @@ -450,29 +543,18 @@ static void timerlat_save_stack(int skip)
> >  	return;
> >  
> >  }
> > -/*
> > - * timerlat_dump_stack - dump a stack trace previously saved
> > - *
> > - * Dump a saved stack trace into the trace buffer.
> > - */
> > -static void timerlat_dump_stack(void)
> > +
> > +static void
> > +__timerlat_dump_stack(struct trace_buffer *buffer, struct trace_stack *fstack, unsigned int size)
> >  {
> >  	struct trace_event_call *call = &event_osnoise;
> > -	struct trace_array *tr = osnoise_trace;
> > -	struct trace_buffer *buffer = tr->array_buffer.buffer;
> >  	struct ring_buffer_event *event;
> > -	struct trace_stack *fstack;
> >  	struct stack_entry *entry;
> > -	unsigned int size;
> > -
> > -	preempt_disable_notrace();
> > -	fstack = this_cpu_ptr(&trace_stack);
> > -	size = fstack->stack_size;
> >  
> >  	event = trace_buffer_lock_reserve(buffer, TRACE_STACK, sizeof(*entry) + size,
> >  					  tracing_gen_ctx());
> >  	if (!event)
> > -		goto out;
> > +		return;
> >  
> >  	entry = ring_buffer_event_data(event);
> >  
> > @@ -481,8 +563,29 @@ static void timerlat_dump_stack(void)
> >  
> >  	if (!call_filter_check_discard(call, entry, buffer, event))
> >  		trace_buffer_unlock_commit_nostack(buffer, event);
> > +}
> >  
> > -out:
> > +/*
> > + * timerlat_dump_stack - dump a stack trace previously saved
> > + */
> > +static void timerlat_dump_stack(void)
> > +{
> > +	struct osnoise_instance *inst;
> > +	struct trace_buffer *buffer;
> > +	struct trace_stack *fstack;
> > +	unsigned int size;
> > +
> > +	preempt_disable_notrace();
> > +	fstack = this_cpu_ptr(&trace_stack);
> > +	size = fstack->stack_size;
> > +
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(inst, &osnoise_instances, list) {
> > +		buffer = inst->tr->array_buffer.buffer;
> > +		__timerlat_dump_stack(buffer, fstack, size);
> > +
> > +	}
> > +	rcu_read_unlock();
> >  	preempt_enable_notrace();
> >  }
> >  #else
> > @@ -1077,12 +1180,37 @@ diff_osn_sample_stats(struct osnoise_variables *osn_var, struct osnoise_sample *
> >   */
> >  static __always_inline void osnoise_stop_tracing(void)
> >  {
> > -	struct trace_array *tr = osnoise_trace;
> > +	struct osnoise_instance *inst;
> > +	struct trace_array *tr;
> > +
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(inst, &osnoise_instances, list) {
> > +		tr = inst->tr;
> > +		trace_array_printk_buf(tr->array_buffer.buffer, _THIS_IP_,
> > +				"stop tracing hit on cpu %d\n", smp_processor_id());
> > +
> > +		tracer_tracing_off(tr);
> > +	}
> > +	rcu_read_unlock();
> > +}
> >  
> > -	trace_array_printk_buf(tr->array_buffer.buffer, _THIS_IP_,
> > -			"stop tracing hit on cpu %d\n", smp_processor_id());
> > +/*
> > + * notify_new_max_latency - Notify a new max latency via fsnotify interface.
> > + */
> > +void notify_new_max_latency(u64 latency)
> > +{
> > +	struct osnoise_instance *inst;
> > +	struct trace_array *tr;
> >  
> > -	tracer_tracing_off(tr);
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(inst, &osnoise_instances, list) {
> > +		tr = inst->tr;
> > +		if (tr->max_latency < latency) {
> > +			tr->max_latency = latency;
> > +			latency_fsnotify(tr);
> > +		}
> > +	}
> > +	rcu_read_unlock();
> >  }
> >  
> >  /*
> > @@ -1096,7 +1224,6 @@ static __always_inline void osnoise_stop_tracing(void)
> >  static int run_osnoise(void)
> >  {
> >  	struct osnoise_variables *osn_var = this_cpu_osn_var();
> > -	struct trace_array *tr = osnoise_trace;
> >  	u64 start, sample, last_sample;
> >  	u64 last_int_count, int_count;
> >  	s64 noise = 0, max_noise = 0;
> > @@ -1231,11 +1358,7 @@ static int run_osnoise(void)
> >  
> >  	trace_osnoise_sample(&s);
> >  
> > -	/* Keep a running maximum ever recorded osnoise "latency" */
> > -	if (max_noise > tr->max_latency) {
> > -		tr->max_latency = max_noise;
> > -		latency_fsnotify(tr);
> > -	}
> > +	notify_new_max_latency(max_noise);
> >  
> >  	if (osnoise_data.stop_tracing_total)
> >  		if (s.noise > osnoise_data.stop_tracing_total)
> > @@ -1293,7 +1416,6 @@ static int osnoise_main(void *data)
> >  static enum hrtimer_restart timerlat_irq(struct hrtimer *timer)
> >  {
> >  	struct osnoise_variables *osn_var = this_cpu_osn_var();
> > -	struct trace_array *tr = osnoise_trace;
> >  	struct timerlat_variables *tlat;
> >  	struct timerlat_sample s;
> >  	u64 now;
> > @@ -1364,11 +1486,7 @@ static enum hrtimer_restart timerlat_irq(struct hrtimer *timer)
> >  
> >  	trace_timerlat_sample(&s);
> >  
> > -	/* Keep a running maximum ever recorded os noise "latency" */
> > -	if (diff > tr->max_latency) {
> > -		tr->max_latency = diff;
> > -		latency_fsnotify(tr);
> > -	}
> > +	notify_new_max_latency(diff);
> >  
> >  	if (osnoise_data.stop_tracing)
> >  		if (time_to_us(diff) >= osnoise_data.stop_tracing)
> > @@ -1581,7 +1699,7 @@ static void osnoise_hotplug_workfn(struct work_struct *dummy)
> >  
> >  	mutex_lock(&trace_types_lock);
> >  
> > -	if (!osnoise_busy)
> > +	if (!osnoise_has_registered_instances())
> >  		goto out_unlock_trace;
> >  
> >  	mutex_lock(&interface_lock);
> > @@ -1716,11 +1834,10 @@ osnoise_cpus_write(struct file *filp, const char __user *ubuf, size_t count,
> >  		goto err_free;
> >  
> >  	/*
> > -	 * trace_types_lock is taken to avoid concurrency on start/stop
> > -	 * and osnoise_busy.
> > +	 * trace_types_lock is taken to avoid concurrency on start/stop.
> >  	 */
> >  	mutex_lock(&trace_types_lock);
> > -	running = osnoise_busy;
> > +	running = osnoise_has_registered_instances();
> >  	if (running)
> >  		stop_per_cpu_kthreads();
> >  
> > @@ -1941,8 +2058,6 @@ static int osnoise_workload_start(void)
> >  		return retval;
> >  	}
> >  
> > -	osnoise_busy = true;
> > -
> >  	return 0;
> >  }
> >  
> > @@ -1951,7 +2066,7 @@ static int osnoise_workload_start(void)
> >   */
> >  static void osnoise_workload_stop(void)
> >  {
> > -	if (!osnoise_busy)
> > +	if (osnoise_has_registered_instances())
> >  		return;
> >  
> >  	trace_osnoise_callback_enabled = false;
> > @@ -1962,28 +2077,28 @@ static void osnoise_workload_stop(void)
> >  	unhook_irq_events();
> >  	unhook_softirq_events();
> >  	unhook_thread_events();
> > -
> > -	osnoise_busy = false;
> >  }
> >  
> >  static void osnoise_tracer_start(struct trace_array *tr)
> >  {
> >  	int retval;
> >  
> > -	if (osnoise_busy)
> > +	if (osnoise_has_registered_instances())
> >  		return;
> >  
> >  	retval = osnoise_workload_start();
> >  	if (retval)
> >  		pr_err(BANNER "Error starting osnoise tracer\n");
> >  
> > +	osnoise_register_instance(tr);
> >  }
> >  
> >  static void osnoise_tracer_stop(struct trace_array *tr)
> >  {
> > -	if (!osnoise_busy)
> > +	if (!osnoise_has_registered_instances())
> >  		return;
> >  
> > +	osnoise_unregister_instance(tr);
> >  	osnoise_workload_stop();
> >  }
> >  
> > @@ -1991,14 +2106,12 @@ static int osnoise_tracer_init(struct trace_array *tr)
> >  {
> >  
> >  	/* Only allow one instance to enable this */
> > -	if (osnoise_busy)
> > +	if (osnoise_has_registered_instances())
> >  		return -EBUSY;
> >  
> > -	osnoise_trace = tr;
> >  	tr->max_latency = 0;
> >  
> >  	osnoise_tracer_start(tr);
> > -
> >  	return 0;
> >  }
> >  
> > @@ -2022,7 +2135,7 @@ static void timerlat_tracer_start(struct trace_array *tr)
> >  {
> >  	int retval;
> >  
> > -	if (osnoise_busy)
> > +	if (osnoise_has_registered_instances())
> >  		return;
> >  
> >  	osnoise_data.timerlat_tracer = 1;
> > @@ -2031,6 +2144,8 @@ static void timerlat_tracer_start(struct trace_array *tr)
> >  	if (retval)
> >  		goto out_err;
> >  
> > +	osnoise_register_instance(tr);
> > +
> >  	return;
> >  out_err:
> >  	pr_err(BANNER "Error starting timerlat tracer\n");
> > @@ -2040,7 +2155,7 @@ static void timerlat_tracer_stop(struct trace_array *tr)
> >  {
> >  	int cpu;
> >  
> > -	if (!osnoise_busy)
> > +	if (!osnoise_has_registered_instances())
> >  		return;
> >  
> >  	for_each_online_cpu(cpu)
> > @@ -2054,11 +2169,9 @@ static void timerlat_tracer_stop(struct trace_array *tr)
> >  static int timerlat_tracer_init(struct trace_array *tr)
> >  {
> >  	/* Only allow one instance to enable this */
> > -	if (osnoise_busy)
> > +	if (osnoise_has_registered_instances())
> >  		return -EBUSY;
> >  
> > -	osnoise_trace = tr;
> > -
> >  	tr->max_latency = 0;
> >  
> >  	timerlat_tracer_start(tr);
> > @@ -2105,6 +2218,8 @@ __init static int init_osnoise_tracer(void)
> >  #endif
> >  	osnoise_init_hotplug_support();
> >  
> > +	INIT_LIST_HEAD_RCU(&osnoise_instances);
> > +
> >  	init_tracefs();
> >  
> >  	return 0;
> 

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