ftrace: comment code

This is first installment of adding documentation to the ftrace.
Expect many more patches of this kind in the near future.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 141 insertions, 1 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 5da391c5fb0d..a102b11eacf2 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -64,26 +64,79 @@ cycle_t ftrace_now(int cpu)
         return cpu_clock(cpu);
 }
 
+/*
+ * The global_trace is the descriptor that holds the tracing
+ * buffers for the live tracing. For each CPU, it contains
+ * a link list of pages that will store trace entries. The
+ * page descriptor of the pages in the memory is used to hold
+ * the link list by linking the lru item in the page descriptor
+ * to each of the pages in the buffer per CPU.
+ *
+ * For each active CPU there is a data field that holds the
+ * pages for the buffer for that CPU. Each CPU has the same number
+ * of pages allocated for its buffer.
+ */
 static struct trace_array       global_trace;
 
 static DEFINE_PER_CPU(struct trace_array_cpu, global_trace_cpu);
 
+/*
+ * The max_tr is used to snapshot the global_trace when a maximum
+ * latency is reached. Some tracers will use this to store a maximum
+ * trace while it continues examining live traces.
+ *
+ * The buffers for the max_tr are set up the same as the global_trace.
+ * When a snapshot is taken, the link list of the max_tr is swapped
+ * with the link list of the global_trace and the buffers are reset for
+ * the global_trace so the tracing can continue.
+ */
 static struct trace_array       max_tr;
 
 static DEFINE_PER_CPU(struct trace_array_cpu, max_data);
 
+/* tracer_enabled is used to toggle activation of a tracer */
 static int                      tracer_enabled = 1;
+
+/*
+ * trace_nr_entries is the number of entries that is allocated
+ * for a buffer. Note, the number of entries is always rounded
+ * to ENTRIES_PER_PAGE.
+ */
 static unsigned long            trace_nr_entries = 65536UL;
 
+/* trace_types holds a link list of available tracers. */
 static struct tracer            *trace_types __read_mostly;
+
+/* current_trace points to the tracer that is currently active */
 static struct tracer            *current_trace __read_mostly;
+
+/*
+ * max_tracer_type_len is used to simplify the allocating of
+ * buffers to read userspace tracer names. We keep track of
+ * the longest tracer name registered.
+ */
 static int                      max_tracer_type_len;
 
+/*
+ * trace_types_lock is used to protect the trace_types list.
+ * This lock is also used to keep user access serialized.
+ * Accesses from userspace will grab this lock while userspace
+ * activities happen inside the kernel.
+ */
 static DEFINE_MUTEX(trace_types_lock);
+
+/* trace_wait is a waitqueue for tasks blocked on trace_poll */
 static DECLARE_WAIT_QUEUE_HEAD(trace_wait);
 
+/* trace_flags holds iter_ctrl options */
 unsigned long trace_flags = TRACE_ITER_PRINT_PARENT;
 
+/**
+ * trace_wake_up - wake up tasks waiting for trace input
+ *
+ * Simply wakes up any task that is blocked on the trace_wait
+ * queue. These is used with trace_poll for tasks polling the trace.
+ */
 void trace_wake_up(void)
 {
         /*
@@ -117,6 +170,14 @@ unsigned long nsecs_to_usecs(unsigned long nsecs)
         return nsecs / 1000;
 }
 
+/*
+ * trace_flag_type is an enumeration that holds different
+ * states when a trace occurs. These are:
+ *  IRQS_OFF    - interrupts were disabled
+ *  NEED_RESCED - reschedule is requested
+ *  HARDIRQ     - inside an interrupt handler
+ *  SOFTIRQ     - inside a softirq handler
+ */
 enum trace_flag_type {
         TRACE_FLAG_IRQS_OFF             = 0x01,
         TRACE_FLAG_NEED_RESCHED         = 0x02,
@@ -124,10 +185,14 @@ enum trace_flag_type {
         TRACE_FLAG_SOFTIRQ              = 0x08,
 };
 
+/*
+ * TRACE_ITER_SYM_MASK masks the options in trace_flags that
+ * control the output of kernel symbols.
+ */
 #define TRACE_ITER_SYM_MASK \
         (TRACE_ITER_PRINT_PARENT|TRACE_ITER_SYM_OFFSET|TRACE_ITER_SYM_ADDR)
 
-/* These must match the bit postions above */
+/* These must match the bit postions in trace_iterator_flags */
 static const char *trace_options[] = {
         "print-parent",
         "sym-offset",
@@ -142,6 +207,15 @@ static const char *trace_options[] = {
         NULL
 };
 
+/*
+ * ftrace_max_lock is used to protect the swapping of buffers
+ * when taking a max snapshot. The buffers themselves are
+ * protected by per_cpu spinlocks. But the action of the swap
+ * needs its own lock.
+ *
+ * This is defined as a raw_spinlock_t in order to help
+ * with performance when lockdep debugging is enabled.
+ */
 static raw_spinlock_t ftrace_max_lock =
         (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
 
@@ -172,6 +246,13 @@ __update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
         tracing_record_cmdline(current);
 }
 
+/**
+ * check_pages - integrity check of trace buffers
+ *
+ * As a safty measure we check to make sure the data pages have not
+ * been corrupted. TODO: configure to disable this because it adds
+ * a bit of overhead.
+ */
 void check_pages(struct trace_array_cpu *data)
 {
         struct page *page, *tmp;
@@ -185,6 +266,13 @@ void check_pages(struct trace_array_cpu *data)
         }
 }
 
+/**
+ * head_page - page address of the first page in per_cpu buffer.
+ *
+ * head_page returns the page address of the first page in
+ * a per_cpu buffer. This also preforms various consistency
+ * checks to make sure the buffer has not been corrupted.
+ */
 void *head_page(struct trace_array_cpu *data)
 {
         struct page *page;
@@ -199,6 +287,17 @@ void *head_page(struct trace_array_cpu *data)
         return page_address(page);
 }
 
+/**
+ * trace_seq_printf - sequence printing of trace information
+ * @s: trace sequence descriptor
+ * @fmt: printf format string
+ *
+ * The tracer may use either sequence operations or its own
+ * copy to user routines. To simplify formating of a trace
+ * trace_seq_printf is used to store strings into a special
+ * buffer (@s). Then the output may be either used by
+ * the sequencer or pulled into another buffer.
+ */