tracing: likely/unlikely branch annotation tracer

Impact: new likely/unlikely branch tracer

This patch adds a way to record the instances of the likely() and unlikely()
branch condition annotations.

When "unlikely" is set in /debugfs/tracing/iter_ctrl the unlikely conditions
will be added to any of the ftrace tracers. The change takes effect when
a new tracer is passed into the current_tracer file.

For example:

 bash-3471  [003]   357.014755: [INCORRECT] sched_info_dequeued:sched_stats.h:177
 bash-3471  [003]   357.014756: [correct] update_curr:sched_fair.c:489
 bash-3471  [003]   357.014758: [correct] calc_delta_fair:sched_fair.c:411
 bash-3471  [003]   357.014759: [correct] account_group_exec_runtime:sched_stats.h:356
 bash-3471  [003]   357.014761: [correct] update_curr:sched_fair.c:489
 bash-3471  [003]   357.014763: [INCORRECT] calc_delta_fair:sched_fair.c:411
 bash-3471  [003]   357.014765: [correct] calc_delta_mine:sched.c:1279

Which shows the normal tracer heading, as well as whether the condition was
correct "[correct]" or was mistaken "[INCORRECT]", followed by the function,
file name and line number.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

5 files changed, 210 insertions, 0 deletions

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index a604f24c755f..8abcaf821beb 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -175,6 +175,28 @@ config TRACE_UNLIKELY_PROFILE
 
           Say N if unsure.
 
+config TRACING_UNLIKELY
+        bool
+        help
+          Selected by tracers that will trace the likely and unlikely
+          conditions. This prevents the tracers themselves from being
+          profiled. Profiling the tracing infrastructure can only happen
+          when the likelys and unlikelys are not being traced.
+
+config UNLIKELY_TRACER
+        bool "Trace likely/unlikely instances"
+        depends on TRACE_UNLIKELY_PROFILE
+        select TRACING_UNLIKELY
+        help
+          This traces the events of likely and unlikely condition
+          calls in the kernel.  The difference between this and the
+          "Trace likely/unlikely profiler" is that this is not a
+          histogram of the callers, but actually places the calling
+          events into a running trace buffer to see when and where the
+          events happened, as well as their results.
+
+          Say N if unsure.
+
 config STACK_TRACER
         bool "Trace max stack"
         depends on HAVE_FUNCTION_TRACER
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 98e70ee27986..c938d03516c0 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -10,6 +10,12 @@ CFLAGS_trace_selftest_dynamic.o = -pg
 obj-y += trace_selftest_dynamic.o
 endif
 
+# If unlikely tracing is enabled, do not trace these files
+ifdef CONFIG_TRACING_UNLIKELY
+KBUILD_CFLAGS += '-Dlikely(x)=likely_notrace(x)'
+KBUILD_CFLAGS += '-Dunlikely(x)=unlikely_notrace(x)'
+endif
+
 obj-$(CONFIG_FUNCTION_TRACER) += libftrace.o
 obj-$(CONFIG_RING_BUFFER) += ring_buffer.o
 
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a3f7ae9cd8e1..83d38634bc90 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -258,6 +258,9 @@ static const char *trace_options[] = {
         "sched-tree",
         "ftrace_printk",
         "ftrace_preempt",
+#ifdef CONFIG_UNLIKELY_TRACER
+        "unlikely",
+#endif
         NULL
 };
 
@@ -1648,6 +1651,18 @@ print_lat_fmt(struct trace_iterator *iter, unsigned int trace_idx, int cpu)
                         trace_seq_print_cont(s, iter);
                 break;
         }
+        case TRACE_UNLIKELY: {
+                struct trace_unlikely *field;
+
+                trace_assign_type(field, entry);
+
+                trace_seq_printf(s, "[%s] %s:%s:%d\n",
+                                 field->correct ? "correct" : "INCORRECT",
+                                 field->func,
+                                 field->file,
+                                 field->line);
+                break;
+        }
         default:
                 trace_seq_printf(s, "Unknown type %d\n", entry->type);
         }
@@ -1787,6 +1802,18 @@ static enum print_line_t print_trace_fmt(struct trace_iterator *iter)
                 return print_return_function(iter);
                 break;
         }
+        case TRACE_UNLIKELY: {
+                struct trace_unlikely *field;
+
+                trace_assign_type(field, entry);
+
+                trace_seq_printf(s, "[%s] %s:%s:%d\n",
+                                 field->correct ? "correct" : "INCORRECT",
+                                 field->func,
+                                 field->file,
+                                 field->line);
+                break;
+        }
         }
         return TRACE_TYPE_HANDLED;
 }
@@ -2592,6 +2619,7 @@ static int tracing_set_tracer(char *buf)
         if (t == current_trace)
                 goto out;
 
+        trace_unlikely_disable();
         if (current_trace && current_trace->reset)
                 current_trace->reset(tr);
 
@@ -2599,6 +2627,7 @@ static int tracing_set_tracer(char *buf)
         if (t->init)
                 t->init(tr);
 
+        trace_unlikely_enable(tr);
  out:
         mutex_unlock(&trace_types_lock);
 
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index b5f91f198fd4..9635aa2c4fc1 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -22,6 +22,7 @@ enum trace_type {
         TRACE_SPECIAL,
         TRACE_MMIO_RW,
         TRACE_MMIO_MAP,
+        TRACE_UNLIKELY,
         TRACE_BOOT_CALL,
         TRACE_BOOT_RET,
         TRACE_FN_RET,
@@ -134,6 +135,16 @@ struct trace_boot_ret {
         struct boot_trace_ret boot_ret;
 };
 
+#define TRACE_FUNC_SIZE 30
+#define TRACE_FILE_SIZE 20
+struct trace_unlikely {
+        struct trace_entry      ent;
+        unsigned                line;
+        char                    func[TRACE_FUNC_SIZE+1];
+        char                    file[TRACE_FILE_SIZE+1];
+        char                    correct;
+};
+
 /*
  * trace_flag_type is an enumeration that holds different
  * states when a trace occurs. These are:
@@ -236,6 +247,7 @@ extern void __ftrace_bad_type(void);
                           TRACE_MMIO_MAP);                              \
                 IF_ASSIGN(var, ent, struct trace_boot_call, TRACE_BOOT_CALL);\
                 IF_ASSIGN(var, ent, struct trace_boot_ret, TRACE_BOOT_RET);\
+                IF_ASSIGN(var, ent, struct trace_unlikely, TRACE_UNLIKELY); \
                 IF_ASSIGN(var, ent, struct ftrace_ret_entry, TRACE_FN_RET);\
                 __ftrace_bad_type();                                    \
         } while (0)
@@ -456,6 +468,9 @@ enum trace_iterator_flags {
         TRACE_ITER_SCHED_TREE           = 0x200,
         TRACE_ITER_PRINTK               = 0x400,
         TRACE_ITER_PREEMPTONLY          = 0x800,
+#ifdef CONFIG_UNLIKELY_TRACER
+        TRACE_ITER_UNLIKELY             = 0x1000,
+#endif
 };
 
 /*
@@ -515,4 +530,28 @@ static inline void ftrace_preempt_enable(int resched)
                 preempt_enable_notrace();
 }
 
+#ifdef CONFIG_UNLIKELY_TRACER
+extern int enable_unlikely_tracing(struct trace_array *tr);
+extern void disable_unlikely_tracing(void);
+static inline int trace_unlikely_enable(struct trace_array *tr)
+{
+        if (trace_flags & TRACE_ITER_UNLIKELY)
+                return enable_unlikely_tracing(tr);
+        return 0;
+}
+static inline void trace_unlikely_disable(void)
+{
+        /* due to races, always disable */
+        disable_unlikely_tracing();
+}
+#else
+static inline int trace_unlikely_enable(struct trace_array *tr)
+{
+        return 0;
+}
+static inline void trace_unlikely_disable(void)
+{
+}
+#endif /* CONFIG_UNLIKELY_TRACER */
+
 #endif /* _LINUX_KERNEL_TRACE_H */
diff --git a/kernel/trace/trace_unlikely.c b/kernel/trace/trace_unlikely.c
index 94932696069f..7290e0e7b4e3 100644
--- a/kernel/trace/trace_unlikely.c
+++ b/kernel/trace/trace_unlikely.c
@@ -15,8 +15,122 @@
 #include <asm/local.h>
 #include "trace.h"
 
+#ifdef CONFIG_UNLIKELY_TRACER
+
+static int unlikely_tracing_enabled __read_mostly;
+static DEFINE_MUTEX(unlikely_tracing_mutex);
+static struct trace_array *unlikely_tracer;
+
+static void
+probe_likely_condition(struct ftrace_likely_data *f, int val, int expect)
+{
+        struct trace_array *tr = unlikely_tracer;
+        struct ring_buffer_event *event;
+        struct trace_unlikely *entry;
+        unsigned long flags, irq_flags;
+        int cpu, pc;
+        const char *p;
+
+        /*
+         * I would love to save just the ftrace_likely_data pointer, but
+         * this code can also be used by modules. Ugly things can happen
+         * if the module is unloaded, and then we go and read the
+         * pointer.  This is slower, but much safer.
+         */
+
+        if (unlikely(!tr))
+                return;
+
+        local_irq_save(flags);
+        cpu = raw_smp_processor_id();
+        if (atomic_inc_return(&tr->data[cpu]->disabled) != 1)
+                goto out;
+
+        event = ring_buffer_lock_reserve(tr->buffer, sizeof(*entry),
+                                         &irq_flags);
+        if (!event)
+                goto out;
+
+        pc = preempt_count();
+        entry   = ring_buffer_event_data(event);
+        tracing_generic_entry_update(&entry->ent, flags, pc);
+        entry->ent.type         = TRACE_UNLIKELY;
+
+        /* Strip off the path, only save the file */
+        p = f->file + strlen(f->file);
+        while (p >= f->file && *p != '/')
+                p--;
+        p++;
+
+        strncpy(entry->func, f->func, TRACE_FUNC_SIZE);
+        strncpy(entry->file, p, TRACE_FILE_SIZE);
+        entry->func[TRACE_FUNC_SIZE] = 0;
+        entry->file[TRACE_FILE_SIZE] = 0;
+        entry->line = f->line;
+        entry->correct = val == expect;
+
+        ring_buffer_unlock_commit(tr->buffer, event, irq_flags);
+
+ out:
+        atomic_dec(&tr->data[cpu]->disabled);
+        local_irq_restore(flags);
+}
+
+static inline
+void trace_likely_condition(struct ftrace_likely_data *f, int val, int expect)
+{
+        if (!unlikely_tracing_enabled)
+                return;
+
+        probe_likely_condition(f, val, expect);
+}
+
+int enable_unlikely_tracing(struct trace_array *tr)
+{
+        int ret = 0;
+
+        mutex_lock(&unlikely_tracing_mutex);
+        unlikely_tracer = tr;
+        /*
+         * Must be seen before enabling. The reader is a condition
+         * where we do not need a matching rmb()
+         */
+        smp_wmb();
+        unlikely_tracing_enabled++;
+        mutex_unlock(&unlikely_tracing_mutex);
+
+        return ret;
+}
+
+void disable_unlikely_tracing(void)
+{
+        mutex_lock(&unlikely_tracing_mutex);
+
+        if (!unlikely_tracing_enabled)
+                goto out_unlock;
+
+        unlikely_tracing_enabled--;
+
+ out_unlock:
+        mutex_unlock(&unlikely_tracing_mutex);
+}
+#else
+static inline
+void trace_likely_condition(struct ftrace_likely_data *f, int val, int expect)
+{
+}
+#endif /* CONFIG_UNLIKELY_TRACER */
+
 void ftrace_likely_update(struct ftrace_likely_data *f, int val, int expect)
 {
+        /*
+         * I would love to have a trace point here instead, but the
+         * trace point code is so inundated with unlikely and likely
+         * conditions that the recursive nightmare that exists is too
+         * much to try to get working. At least for now.
+         */
+        trace_likely_condition(f, val, expect);
+
         /* FIXME: Make this atomic! */
         if (val == expect)
                 f->correct++;