Date:   Mon, 25 Oct 2021 11:10:26 +0200
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     paulmck@...nel.org, Steven Rostedt <rostedt@...dmis.org>
Cc:     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 10/25/21 05:26, Paul E. McKenney wrote:
> 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

Thanks Steven and Paul!

The start/stop/reset operations are called from trace "core" operations. So they
are all protected/serialized by trace_types_lock (by trace/core).

So, it seems that the way to go is to remove the rcu_read_lock/unlock() from
unregister, adding a lockdep check to see if trace_types_lock is help as a bonus?

-- Daniel

> 
>>> +			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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