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

From: Paul E. McKenney
Date: Mon Oct 25 2021 - 10:33:38 EST

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In reply to: Steven Rostedt: "Re: [PATCH V4 04/19] trace/osnoise: Support a list of trace_array *tr"
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On Mon, Oct 25, 2021 at 11:10:26AM +0200, Daniel Bristot de Oliveira wrote:
> 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@xxxxxxxxxx> 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@xxxxxxxxxxx>
> >>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >>> Cc: Tom Zanussi <zanussi@xxxxxxxxxx>
> >>> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> >>> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> >>> Cc: Clark Williams <williams@xxxxxxxxxx>
> >>> Cc: John Kacur <jkacur@xxxxxxxxxx>
> >>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >>> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> >>> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> >>> Cc: linux-rt-users@xxxxxxxxxxxxxxx
> >>> Cc: linux-trace-devel@xxxxxxxxxxxxxxx
> >>> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >>> Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> >>> ---
> >>> 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?

Works for me!

Thanx, Paul

> -- 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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In reply to: Steven Rostedt: "Re: [PATCH V4 04/19] trace/osnoise: Support a list of trace_array *tr"
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