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Subject[PATCH 2/2 v2] tracing: Add tp_printk cmdline to have tracepoints go to printk()
From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

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: http://lkml.kernel.org/r/alpine.DEB.2.11.1412121539300.16494@nanos

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
Documentation/kernel-parameters.txt | 18 ++++++++++++++++++
include/linux/ftrace.h | 1 +
kernel/sysctl.c | 7 +++++++
kernel/trace/trace.c | 17 +++++++++++++++++
kernel/trace/trace.h | 1 +
kernel/trace/trace_events.c | 32 ++++++++++++++++++++++++++++++++
6 files changed, 76 insertions(+)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 1d09eb37c562..ae41f1181e9a 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3500,6 +3500,24 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
See also Documentation/trace/ftrace.txt "trace options"
section.

+ tp_printk[FTRACE]
+ 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
+ ftrace_dump_on_oops.
+
+ 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.
+
+ ** CAUTION **
+
+ Having tracepoints sent to printk() and activating high
+ frequency tracepoints such as irq or sched, can cause
+ the system to live lock.
+
traceoff_on_warning
[FTRACE] enable this option to disable tracing when a
warning is hit. This turns off "tracing_on". Tracing can
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index f4bc14b7d444..1da602982cf9 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -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;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 4aada6d9fe74..bb50c2187194 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -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,
+ },
#endif
#ifdef CONFIG_KEXEC
{
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ec3ca694665f..e890d2d4ec89 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -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;
+ }
tracer_alloc_buffers();
init_ftrace_syscalls();
trace_event_init();
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index c138c149d6ef..8de48bac1ce2 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1313,5 +1313,6 @@ void trace_event_init(void);
static inline void __init trace_event_init(void) { }
#endif

+extern struct trace_iterator *tracepoint_print_iter;

#endif /* _LINUX_KERNEL_TRACE_H */
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index fd9deb0e03f0..9f7175a3df71 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -212,8 +212,40 @@ void *ftrace_event_buffer_reserve(struct ftrace_event_buffer *fbuffer,
}
EXPORT_SYMBOL_GPL(ftrace_event_buffer_reserve);

+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)
+ return;
+
+ event_call = fbuffer->ftrace_file->event_call;
+ if (!event_call || !event_call->event.funcs ||
+ !event_call->event.funcs->trace)
+ return;
+
+ event = &fbuffer->ftrace_file->event_call->event;
+
+ spin_lock_irqsave(&tracepoint_iter_lock, flags);
+ trace_seq_init(&iter->seq);
+ 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)
+ output_printk(fbuffer);
+
event_trigger_unlock_commit(fbuffer->ftrace_file, fbuffer->buffer,
fbuffer->event, fbuffer->entry,
fbuffer->flags, fbuffer->pc);
--
2.1.3



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