Commit 0daa2302 authored by Steven Rostedt (Red Hat)'s avatar Steven Rostedt (Red Hat) Committed by Steven Rostedt

tracing: Add tp_printk cmdline to have tracepoints go to printk()

Add the kernel command line tp_printk option that will have tracepoints
that are active sent to printk() as well as to the trace buffer.

Passing "tp_printk" will activate this. To turn it off, the sysctl
/proc/sys/kernel/tracepoint_printk can have '0' echoed into it. Note,
this only works if the cmdline option is used. Echoing 1 into the sysctl
file without the cmdline option will have no affect.

Note, this is a dangerous option. Having high frequency tracepoints send
their data to printk() can possibly cause a live lock. This is another
reason why this is only active if the command line option is used.

Link: default avatarThomas Gleixner <>
Tested-by: default avatarThomas Gleixner <>
Acked-by: default avatarThomas Gleixner <>
Signed-off-by: default avatarSteven Rostedt <>
parent 5f893b26
......@@ -3500,6 +3500,24 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
See also Documentation/trace/ftrace.txt "trace options"
Have the tracepoints sent to printk as well as the
tracing ring buffer. This is useful for early boot up
where the system hangs or reboots and does not give the
option for reading the tracing buffer or performing a
To turn off having tracepoints sent to printk,
echo 0 > /proc/sys/kernel/tracepoint_printk
Note, echoing 1 into this file without the
tracepoint_printk kernel cmdline option has no effect.
Having tracepoints sent to printk() and activating high
frequency tracepoints such as irq or sched, can cause
the system to live lock.
[FTRACE] enable this option to disable tracing when a
warning is hit. This turns off "tracing_on". Tracing can
......@@ -879,6 +879,7 @@ static inline int test_tsk_trace_graph(struct task_struct *tsk)
enum ftrace_dump_mode;
extern enum ftrace_dump_mode ftrace_dump_on_oops;
extern int tracepoint_printk;
extern void disable_trace_on_warning(void);
extern int __disable_trace_on_warning;
......@@ -622,6 +622,13 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec,
.procname = "tracepoint_printk",
.data = &tracepoint_printk,
.maxlen = sizeof(tracepoint_printk),
.mode = 0644,
.proc_handler = proc_dointvec,
......@@ -63,6 +63,10 @@ static bool __read_mostly tracing_selftest_running;
bool __read_mostly tracing_selftest_disabled;
/* Pipe tracepoints to printk */
struct trace_iterator *tracepoint_print_iter;
int tracepoint_printk;
/* For tracers that don't implement custom flags */
static struct tracer_opt dummy_tracer_opt[] = {
{ }
......@@ -193,6 +197,13 @@ static int __init set_trace_boot_clock(char *str)
__setup("trace_clock=", set_trace_boot_clock);
static int __init set_tracepoint_printk(char *str)
if ((strcmp(str, "=0") != 0 && strcmp(str, "=off") != 0))
tracepoint_printk = 1;
return 1;
__setup("tp_printk", set_tracepoint_printk);
unsigned long long ns2usecs(cycle_t nsec)
......@@ -6878,6 +6889,12 @@ out:
void __init trace_init(void)
if (tracepoint_printk) {
tracepoint_print_iter =
kmalloc(sizeof(*tracepoint_print_iter), GFP_KERNEL);
if (WARN_ON(!tracepoint_print_iter))
tracepoint_printk = 0;
......@@ -1313,5 +1313,6 @@ void trace_event_init(void);
static inline void __init trace_event_init(void) { }
extern struct trace_iterator *tracepoint_print_iter;
#endif /* _LINUX_KERNEL_TRACE_H */
......@@ -212,8 +212,40 @@ void *ftrace_event_buffer_reserve(struct ftrace_event_buffer *fbuffer,
static DEFINE_SPINLOCK(tracepoint_iter_lock);
static void output_printk(struct ftrace_event_buffer *fbuffer)
struct ftrace_event_call *event_call;
struct trace_event *event;
unsigned long flags;
struct trace_iterator *iter = tracepoint_print_iter;
if (!iter)
event_call = fbuffer->ftrace_file->event_call;
if (!event_call || !event_call->event.funcs ||
event = &fbuffer->ftrace_file->event_call->event;
spin_lock_irqsave(&tracepoint_iter_lock, flags);
iter->ent = fbuffer->entry;
event_call->event.funcs->trace(iter, 0, event);
trace_seq_putc(&iter->seq, 0);
printk("%s", iter->seq.buffer);
spin_unlock_irqrestore(&tracepoint_iter_lock, flags);
void ftrace_event_buffer_commit(struct ftrace_event_buffer *fbuffer)
if (tracepoint_printk)
event_trigger_unlock_commit(fbuffer->ftrace_file, fbuffer->buffer,
fbuffer->event, fbuffer->entry,
fbuffer->flags, fbuffer->pc);
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