When debugging device drivers, I've found it very useful to be able to
redirect existing pr_debug()/dev_dbg() prints to the trace buffer
instead of dmesg. Among the many advantages of the trace buffer is that
it can be dynamically resized, allows these prints to combined with
other trace events, and doesn't fill up system logs.
Since dynamic debug already has hooks in these call sites, getting these
prints into the ftrace buffer is straightforward if we have dynamic
debug do it.
Add an "x" flag to make the dynamic debug call site print to a new
printk:dynamic trace event. The trace event can be emitted instead of
or in addition to the printk().
The print buffer is statically allocated and managed using code borrowed
from __ftrace_trace_stack() and is limited to 256 bytes (four of these
are allocated per CPU to handle the various contexts); anything larger
will be truncated.
Signed-off-by: Vincent Whitchurch <[email protected]>
---
v2:
- Remove stack buffer and use code similar to __ftrace_trace_stack()
- Use an event with the same class as printk:console
.../admin-guide/dynamic-debug-howto.rst | 1 +
include/linux/dynamic_debug.h | 7 +-
include/trace/events/printk.h | 12 +-
lib/dynamic_debug.c | 155 +++++++++++++++---
4 files changed, 146 insertions(+), 29 deletions(-)
diff --git a/Documentation/admin-guide/dynamic-debug-howto.rst b/Documentation/admin-guide/dynamic-debug-howto.rst
index e5a8def45f3f..d2ebee464db7 100644
--- a/Documentation/admin-guide/dynamic-debug-howto.rst
+++ b/Documentation/admin-guide/dynamic-debug-howto.rst
@@ -229,6 +229,7 @@ of the characters::
The flags are::
p enables the pr_debug() callsite.
+ x enables trace to the printk:dynamic event
f Include the function name in the printed message
l Include line number in the printed message
m Include module name in the printed message
diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index aa9ff9e1c0b3..f599ed21ecc5 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -27,13 +27,16 @@ struct _ddebug {
* writes commands to <debugfs>/dynamic_debug/control
*/
#define _DPRINTK_FLAGS_NONE 0
-#define _DPRINTK_FLAGS_PRINT (1<<0) /* printk() a message using the format */
+#define _DPRINTK_FLAGS_PRINTK (1<<0) /* printk() a message using the format */
#define _DPRINTK_FLAGS_INCL_MODNAME (1<<1)
#define _DPRINTK_FLAGS_INCL_FUNCNAME (1<<2)
#define _DPRINTK_FLAGS_INCL_LINENO (1<<3)
#define _DPRINTK_FLAGS_INCL_TID (1<<4)
+#define _DPRINTK_FLAGS_TRACE (1<<5)
+#define _DPRINTK_FLAGS_PRINT (_DPRINTK_FLAGS_PRINTK | \
+ _DPRINTK_FLAGS_TRACE)
#if defined DEBUG
-#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINT
+#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINTK
#else
#define _DPRINTK_FLAGS_DEFAULT 0
#endif
diff --git a/include/trace/events/printk.h b/include/trace/events/printk.h
index 13d405b2fd8b..6c89121a1669 100644
--- a/include/trace/events/printk.h
+++ b/include/trace/events/printk.h
@@ -7,7 +7,7 @@
#include <linux/tracepoint.h>
-TRACE_EVENT(console,
+DECLARE_EVENT_CLASS(printk,
TP_PROTO(const char *text, size_t len),
TP_ARGS(text, len),
@@ -31,6 +31,16 @@ TRACE_EVENT(console,
TP_printk("%s", __get_str(msg))
);
+
+DEFINE_EVENT(printk, console,
+ TP_PROTO(const char *text, size_t len),
+ TP_ARGS(text, len)
+);
+
+DEFINE_EVENT(printk, dynamic,
+ TP_PROTO(const char *text, size_t len),
+ TP_ARGS(text, len)
+);
#endif /* _TRACE_PRINTK_H */
/* This part must be outside protection */
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 1d012e597cc3..76fc3e33fe41 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -36,6 +36,7 @@
#include <linux/sched.h>
#include <linux/device.h>
#include <linux/netdevice.h>
+#include <trace/events/printk.h>
#include <rdma/ib_verbs.h>
@@ -84,11 +85,12 @@ static inline const char *trim_prefix(const char *path)
}
static struct { unsigned flag:8; char opt_char; } opt_array[] = {
- { _DPRINTK_FLAGS_PRINT, 'p' },
+ { _DPRINTK_FLAGS_PRINTK, 'p' },
{ _DPRINTK_FLAGS_INCL_MODNAME, 'm' },
{ _DPRINTK_FLAGS_INCL_FUNCNAME, 'f' },
{ _DPRINTK_FLAGS_INCL_LINENO, 'l' },
{ _DPRINTK_FLAGS_INCL_TID, 't' },
+ { _DPRINTK_FLAGS_TRACE, 'x' },
{ _DPRINTK_FLAGS_NONE, '_' },
};
@@ -600,6 +602,96 @@ static char *dynamic_emit_prefix(const struct _ddebug *desc, char *buf)
return buf;
}
+/*
+ * This code is heavily based on __ftrace_trace_stack().
+ *
+ * Allow 4 levels of nesting: normal, softirq, irq, NMI.
+ */
+#define DYNAMIC_TRACE_NESTING 4
+
+struct dynamic_trace_buf {
+ char buf[256];
+};
+
+struct dynamic_trace_bufs {
+ struct dynamic_trace_buf bufs[DYNAMIC_TRACE_NESTING];
+};
+
+static DEFINE_PER_CPU(struct dynamic_trace_bufs, dynamic_trace_bufs);
+static DEFINE_PER_CPU(int, dynamic_trace_reserve);
+
+static void dynamic_trace(const char *fmt, va_list args)
+{
+ struct dynamic_trace_buf *buf;
+ int bufidx;
+ int len;
+
+ preempt_disable_notrace();
+
+ bufidx = __this_cpu_inc_return(dynamic_trace_reserve) - 1;
+
+ if (WARN_ON_ONCE(bufidx > DYNAMIC_TRACE_NESTING))
+ goto out;
+
+ /* For the same reasons as in __ftrace_trace_stack(). */
+ barrier();
+
+ buf = this_cpu_ptr(dynamic_trace_bufs.bufs) + bufidx;
+
+ len = vscnprintf(buf->buf, sizeof(buf->buf), fmt, args);
+ trace_dynamic(buf->buf, len);
+
+out:
+ /* As above. */
+ barrier();
+ __this_cpu_dec(dynamic_trace_reserve);
+ preempt_enable_notrace();
+}
+
+static void dynamic_printk(unsigned int flags, const char *fmt, ...)
+{
+ if (flags & _DPRINTK_FLAGS_TRACE) {
+ va_list args;
+
+ va_start(args, fmt);
+ /*
+ * All callers include the KERN_DEBUG prefix to keep the
+ * vprintk case simple; strip it out for tracing.
+ */
+ dynamic_trace(fmt + strlen(KERN_DEBUG), args);
+ va_end(args);
+ }
+
+ if (flags & _DPRINTK_FLAGS_PRINTK) {
+ va_list args;
+
+ va_start(args, fmt);
+ vprintk(fmt, args);
+ va_end(args);
+ }
+}
+
+static void dynamic_dev_printk(unsigned int flags, const struct device *dev,
+ const char *fmt, ...)
+{
+
+ if (flags & _DPRINTK_FLAGS_TRACE) {
+ va_list args;
+
+ va_start(args, fmt);
+ dynamic_trace(fmt, args);
+ va_end(args);
+ }
+
+ if (flags & _DPRINTK_FLAGS_PRINTK) {
+ va_list args;
+
+ va_start(args, fmt);
+ dev_vprintk_emit(LOGLEVEL_DEBUG, dev, fmt, args);
+ va_end(args);
+ }
+}
+
void __dynamic_pr_debug(struct _ddebug *descriptor, const char *fmt, ...)
{
va_list args;
@@ -614,7 +706,8 @@ void __dynamic_pr_debug(struct _ddebug *descriptor, const char *fmt, ...)
vaf.fmt = fmt;
vaf.va = &args;
- printk(KERN_DEBUG "%s%pV", dynamic_emit_prefix(descriptor, buf), &vaf);
+ dynamic_printk(descriptor->flags, KERN_DEBUG "%s%pV",
+ dynamic_emit_prefix(descriptor, buf), &vaf);
va_end(args);
}
@@ -624,6 +717,7 @@ void __dynamic_dev_dbg(struct _ddebug *descriptor,
const struct device *dev, const char *fmt, ...)
{
struct va_format vaf;
+ unsigned int flags;
va_list args;
BUG_ON(!descriptor);
@@ -633,16 +727,18 @@ void __dynamic_dev_dbg(struct _ddebug *descriptor,
vaf.fmt = fmt;
vaf.va = &args;
+ flags = descriptor->flags;
if (!dev) {
- printk(KERN_DEBUG "(NULL device *): %pV", &vaf);
+ dynamic_printk(flags, KERN_DEBUG "(NULL device *): %pV",
+ &vaf);
} else {
char buf[PREFIX_SIZE];
- dev_printk_emit(LOGLEVEL_DEBUG, dev, "%s%s %s: %pV",
- dynamic_emit_prefix(descriptor, buf),
- dev_driver_string(dev), dev_name(dev),
- &vaf);
+ dynamic_dev_printk(flags, dev, "%s%s %s: %pV",
+ dynamic_emit_prefix(descriptor, buf),
+ dev_driver_string(dev), dev_name(dev),
+ &vaf);
}
va_end(args);
@@ -655,6 +751,7 @@ void __dynamic_netdev_dbg(struct _ddebug *descriptor,
const struct net_device *dev, const char *fmt, ...)
{
struct va_format vaf;
+ unsigned int flags;
va_list args;
BUG_ON(!descriptor);
@@ -664,22 +761,24 @@ void __dynamic_netdev_dbg(struct _ddebug *descriptor,
vaf.fmt = fmt;
vaf.va = &args;
+ flags = descriptor->flags;
if (dev && dev->dev.parent) {
char buf[PREFIX_SIZE];
- dev_printk_emit(LOGLEVEL_DEBUG, dev->dev.parent,
- "%s%s %s %s%s: %pV",
- dynamic_emit_prefix(descriptor, buf),
- dev_driver_string(dev->dev.parent),
- dev_name(dev->dev.parent),
- netdev_name(dev), netdev_reg_state(dev),
- &vaf);
+ dynamic_dev_printk(flags, dev->dev.parent,
+ "%s%s %s %s%s: %pV",
+ dynamic_emit_prefix(descriptor, buf),
+ dev_driver_string(dev->dev.parent),
+ dev_name(dev->dev.parent),
+ netdev_name(dev), netdev_reg_state(dev),
+ &vaf);
} else if (dev) {
- printk(KERN_DEBUG "%s%s: %pV", netdev_name(dev),
- netdev_reg_state(dev), &vaf);
+ dynamic_printk(flags, KERN_DEBUG "%s%s: %pV",
+ netdev_name(dev), netdev_reg_state(dev), &vaf);
} else {
- printk(KERN_DEBUG "(NULL net_device): %pV", &vaf);
+ dynamic_printk(flags, KERN_DEBUG "(NULL net_device): %pV",
+ &vaf);
}
va_end(args);
@@ -694,27 +793,31 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
const struct ib_device *ibdev, const char *fmt, ...)
{
struct va_format vaf;
+ unsigned int flags;
va_list args;
va_start(args, fmt);
vaf.fmt = fmt;
vaf.va = &args;
+ flags = descriptor->flags;
if (ibdev && ibdev->dev.parent) {
char buf[PREFIX_SIZE];
- dev_printk_emit(LOGLEVEL_DEBUG, ibdev->dev.parent,
- "%s%s %s %s: %pV",
- dynamic_emit_prefix(descriptor, buf),
- dev_driver_string(ibdev->dev.parent),
- dev_name(ibdev->dev.parent),
- dev_name(&ibdev->dev),
- &vaf);
+ dynamic_dev_printk(flags, ibdev->dev.parent,
+ "%s%s %s %s: %pV",
+ dynamic_emit_prefix(descriptor, buf),
+ dev_driver_string(ibdev->dev.parent),
+ dev_name(ibdev->dev.parent),
+ dev_name(&ibdev->dev),
+ &vaf);
} else if (ibdev) {
- printk(KERN_DEBUG "%s: %pV", dev_name(&ibdev->dev), &vaf);
+ dynamic_printk(flags, KERN_DEBUG "%s: %pV",
+ dev_name(&ibdev->dev), &vaf);
} else {
- printk(KERN_DEBUG "(NULL ib_device): %pV", &vaf);
+ dynamic_printk(flags, KERN_DEBUG "(NULL ib_device): %pV",
+ &vaf);
}
va_end(args);
--
2.25.1
On Fri, 14 Aug 2020 15:31:51 +0200
Vincent Whitchurch <[email protected]> wrote:
> index aa9ff9e1c0b3..f599ed21ecc5 100644
> --- a/include/linux/dynamic_debug.h
> +++ b/include/linux/dynamic_debug.h
> @@ -27,13 +27,16 @@ struct _ddebug {
> * writes commands to <debugfs>/dynamic_debug/control
> */
> #define _DPRINTK_FLAGS_NONE 0
> -#define _DPRINTK_FLAGS_PRINT (1<<0) /* printk() a message using the format */
> +#define _DPRINTK_FLAGS_PRINTK (1<<0) /* printk() a message using the format */
The above looks like a cleanup unrelated to this patch, and probably
should be on its own.
> #define _DPRINTK_FLAGS_INCL_MODNAME (1<<1)
> #define _DPRINTK_FLAGS_INCL_FUNCNAME (1<<2)
> #define _DPRINTK_FLAGS_INCL_LINENO (1<<3)
> #define _DPRINTK_FLAGS_INCL_TID (1<<4)
> +#define _DPRINTK_FLAGS_TRACE (1<<5)
> +#define _DPRINTK_FLAGS_PRINT (_DPRINTK_FLAGS_PRINTK | \
> + _DPRINTK_FLAGS_TRACE)
> #if defined DEBUG
> -#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINT
> +#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINTK
> #else
> #define _DPRINTK_FLAGS_DEFAULT 0
> #endif
> diff --git a/include/trace/events/printk.h b/include/trace/events/printk.h
> index 13d405b2fd8b..6c89121a1669 100644
> --- a/include/trace/events/printk.h
> +++ b/include/trace/events/printk.h
> @@ -7,7 +7,7 @@
>
> #include <linux/tracepoint.h>
>
> -TRACE_EVENT(console,
> +DECLARE_EVENT_CLASS(printk,
> TP_PROTO(const char *text, size_t len),
>
> TP_ARGS(text, len),
> @@ -31,6 +31,16 @@ TRACE_EVENT(console,
>
> TP_printk("%s", __get_str(msg))
> );
> +
> +DEFINE_EVENT(printk, console,
> + TP_PROTO(const char *text, size_t len),
> + TP_ARGS(text, len)
> +);
> +
> +DEFINE_EVENT(printk, dynamic,
Can we call this "dynamic_printk" or "printk_dynamic", as
trace_dynamic() is too generic.
> + TP_PROTO(const char *text, size_t len),
> + TP_ARGS(text, len)
> +);
> #endif /* _TRACE_PRINTK_H */
>
> /* This part must be outside protection */
> diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
> index 1d012e597cc3..76fc3e33fe41 100644
> --- a/lib/dynamic_debug.c
> +++ b/lib/dynamic_debug.c
> @@ -36,6 +36,7 @@
> #include <linux/sched.h>
> #include <linux/device.h>
> #include <linux/netdevice.h>
> +#include <trace/events/printk.h>
>
> #include <rdma/ib_verbs.h>
>
> @@ -84,11 +85,12 @@ static inline const char *trim_prefix(const char *path)
> }
>
> static struct { unsigned flag:8; char opt_char; } opt_array[] = {
> - { _DPRINTK_FLAGS_PRINT, 'p' },
> + { _DPRINTK_FLAGS_PRINTK, 'p' },
Again, this looks unrelated, and shouldn't be part of this patch.
> { _DPRINTK_FLAGS_INCL_MODNAME, 'm' },
> { _DPRINTK_FLAGS_INCL_FUNCNAME, 'f' },
> { _DPRINTK_FLAGS_INCL_LINENO, 'l' },
> { _DPRINTK_FLAGS_INCL_TID, 't' },
> + { _DPRINTK_FLAGS_TRACE, 'x' },
> { _DPRINTK_FLAGS_NONE, '_' },
> };
>
Other then the two comments above, I'm fine with this change.
Reviewed-by: Steven Rostedt (VMware) <[email protected]>
-- Steve
On 8/14/20 1:15 PM, Steven Rostedt wrote:
> On Fri, 14 Aug 2020 15:31:51 +0200
> Vincent Whitchurch <[email protected]> wrote:
>> index aa9ff9e1c0b3..f599ed21ecc5 100644
>> --- a/include/linux/dynamic_debug.h
>> +++ b/include/linux/dynamic_debug.h
>> @@ -27,13 +27,16 @@ struct _ddebug {
>> * writes commands to <debugfs>/dynamic_debug/control
>> */
>> #define _DPRINTK_FLAGS_NONE 0
>> -#define _DPRINTK_FLAGS_PRINT (1<<0) /* printk() a message using the format */
>> +#define _DPRINTK_FLAGS_PRINTK (1<<0) /* printk() a message using the format */
>
> The above looks like a cleanup unrelated to this patch, and probably
> should be on its own.
I read it as we used to have this one thing called 'print', which really meant
printk, but now that we also have the ability to output to the trace buffer,
what does 'print' mean now? So I read it as being part of this change.
>
>> #define _DPRINTK_FLAGS_INCL_MODNAME (1<<1)
>> #define _DPRINTK_FLAGS_INCL_FUNCNAME (1<<2)
>> #define _DPRINTK_FLAGS_INCL_LINENO (1<<3)
>> #define _DPRINTK_FLAGS_INCL_TID (1<<4)
>> +#define _DPRINTK_FLAGS_TRACE (1<<5)
>> +#define _DPRINTK_FLAGS_PRINT (_DPRINTK_FLAGS_PRINTK | \
>> + _DPRINTK_FLAGS_TRACE)
Is _DPRINTK_FLAGS_PRINT actually used anywhere? Looks to me like
it can be removed.
This is a feature I've wanted for dynamic debug for a while. Thanks for
implementing it!
Dynamic can be enabled on the command line in order to print things early
in boot (I think ftrace can as well), I want to make sure that there are
no ordering issues here? And things wouldn't blow up if we enable printing
to the ftrace buffer early on via dyanmic debug?
Thanks,
-Jason
On Fri, Aug 14, 2020 at 11:30:34PM +0200, Jason Baron wrote:
> On 8/14/20 1:15 PM, Steven Rostedt wrote:
> > On Fri, 14 Aug 2020 15:31:51 +0200
> > Vincent Whitchurch <[email protected]> wrote:
> >> index aa9ff9e1c0b3..f599ed21ecc5 100644
> >> --- a/include/linux/dynamic_debug.h
> >> +++ b/include/linux/dynamic_debug.h
> >> @@ -27,13 +27,16 @@ struct _ddebug {
> >> * writes commands to <debugfs>/dynamic_debug/control
> >> */
> >> #define _DPRINTK_FLAGS_NONE 0
> >> -#define _DPRINTK_FLAGS_PRINT (1<<0) /* printk() a message using the format */
> >> +#define _DPRINTK_FLAGS_PRINTK (1<<0) /* printk() a message using the format */
> >
> > The above looks like a cleanup unrelated to this patch, and probably
> > should be on its own.
>
> I read it as we used to have this one thing called 'print', which really meant
> printk, but now that we also have the ability to output to the trace buffer,
> what does 'print' mean now? So I read it as being part of this change.
Yes, that's what was intended, but I think it makes sense to split it
out as Steven suggested so I've done that now (and also renamed the
combined flag to the less ambiguous _DPRINTK_FLAGS_ENABLE).
>
> >
> >> #define _DPRINTK_FLAGS_INCL_MODNAME (1<<1)
> >> #define _DPRINTK_FLAGS_INCL_FUNCNAME (1<<2)
> >> #define _DPRINTK_FLAGS_INCL_LINENO (1<<3)
> >> #define _DPRINTK_FLAGS_INCL_TID (1<<4)
> >> +#define _DPRINTK_FLAGS_TRACE (1<<5)
> >> +#define _DPRINTK_FLAGS_PRINT (_DPRINTK_FLAGS_PRINTK | \
> >> + _DPRINTK_FLAGS_TRACE)
>
>
> Is _DPRINTK_FLAGS_PRINT actually used anywhere? Looks to me like
> it can be removed.
It's used from DYNAMIC_DEBUG_BRANCH() as well as from
lib/dynamic_debug.c to check if the location is enabled.
> This is a feature I've wanted for dynamic debug for a while. Thanks for
> implementing it!
>
> Dynamic can be enabled on the command line in order to print things early
> in boot (I think ftrace can as well), I want to make sure that there are
> no ordering issues here? And things wouldn't blow up if we enable printing
> to the ftrace buffer early on via dyanmic debug?
I tried enabling all dynamic debug locations and tracing via the command
line and that worked fine:
dyndbg="file * +x" trace_event=printk:*
On Fri, Aug 14, 2020 at 07:15:31PM +0200, Steven Rostedt wrote:
> On Fri, 14 Aug 2020 15:31:51 +0200
> Vincent Whitchurch <[email protected]> wrote:
> > index aa9ff9e1c0b3..f599ed21ecc5 100644
> > --- a/include/linux/dynamic_debug.h
> > +++ b/include/linux/dynamic_debug.h
> > @@ -27,13 +27,16 @@ struct _ddebug {
> > * writes commands to <debugfs>/dynamic_debug/control
> > */
> > #define _DPRINTK_FLAGS_NONE 0
> > -#define _DPRINTK_FLAGS_PRINT (1<<0) /* printk() a message using the format */
> > +#define _DPRINTK_FLAGS_PRINTK (1<<0) /* printk() a message using the format */
>
> The above looks like a cleanup unrelated to this patch, and probably
> should be on its own.
I've moved this and the other renaming hunk out to a separate patch.
> > #define _DPRINTK_FLAGS_INCL_MODNAME (1<<1)
> > #define _DPRINTK_FLAGS_INCL_FUNCNAME (1<<2)
> > #define _DPRINTK_FLAGS_INCL_LINENO (1<<3)
> > #define _DPRINTK_FLAGS_INCL_TID (1<<4)
> > +#define _DPRINTK_FLAGS_TRACE (1<<5)
> > +#define _DPRINTK_FLAGS_PRINT (_DPRINTK_FLAGS_PRINTK | \
> > + _DPRINTK_FLAGS_TRACE)
> > #if defined DEBUG
> > -#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINT
> > +#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINTK
> > #else
> > #define _DPRINTK_FLAGS_DEFAULT 0
> > #endif
> > diff --git a/include/trace/events/printk.h b/include/trace/events/printk.h
> > index 13d405b2fd8b..6c89121a1669 100644
> > --- a/include/trace/events/printk.h
> > +++ b/include/trace/events/printk.h
> > @@ -7,7 +7,7 @@
> >
> > #include <linux/tracepoint.h>
> >
> > -TRACE_EVENT(console,
> > +DECLARE_EVENT_CLASS(printk,
> > TP_PROTO(const char *text, size_t len),
> >
> > TP_ARGS(text, len),
> > @@ -31,6 +31,16 @@ TRACE_EVENT(console,
> >
> > TP_printk("%s", __get_str(msg))
> > );
> > +
> > +DEFINE_EVENT(printk, console,
> > + TP_PROTO(const char *text, size_t len),
> > + TP_ARGS(text, len)
> > +);
> > +
> > +DEFINE_EVENT(printk, dynamic,
>
> Can we call this "dynamic_printk" or "printk_dynamic", as
> trace_dynamic() is too generic.
I went for "dyndbg" (printk:dyndbg / trace_dyndbg()) which is the
documented name of the infrastructure.