Re: [PATCH V4 04/19] trace/osnoise: Support a list of trace_array *tr

Paul E. McKenney
2021-10-25T03:26:16Z

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@kernel.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@goodmis.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Tom Zanussi <zanussi@kernel.org>
> > Cc: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: Juri Lelli <juri.lelli@redhat.com>
> > Cc: Clark Williams <williams@redhat.com>
> > Cc: John Kacur <jkacur@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
> > Cc: linux-rt-users@vger.kernel.org
> > Cc: linux-trace-devel@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.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;
>