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tracing: Add a way to soft disable trace events

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In order to let triggers enable or disable events, we need a 'soft'
method for doing so. For example, if a function probe is added that
lets a user enable or disable events when a function is called, that
change must be done without taking locks or a mutex, and definitely
it can't sleep. But the full enabling of a tracepoint is expensive.

By adding a 'SOFT_DISABLE' flag, and converting the flags to be updated
without the protection of a mutex (using set/clear_bit()), this soft
disable flag can be used to allow critical sections to enable or disable
events from being traced (after the event has been placed into "SOFT_MODE").

Some caveats though: The comm recorder (to map pids with a comm) can not
be soft disabled (yet). If you disable an event with with a "soft"
disable and wait a while before reading the trace, the comm cache may be
replaced and you'll get a bunch of <...> for comms in the trace.

Reading the "enable" file for an event that is disabled will now give
you "0*" where the '*' denotes that the tracepoint is still active but
the event itself is "disabled".

[ fixed _BIT used in & operation : thanks to Dan Carpenter and smatch ]

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (Red Hat) 2013-03-12 13:26:18 -04:00 • committed by Steven Rostedt
parent 7818b38865
commit 417944c4c7
3 changed files with 84 additions and 19 deletions
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20
include/linux/ftrace_event.h
View file

@ -251,16 +251,23 @@ struct ftrace_subsystem_dir;
enum {
FTRACE_EVENT_FL_ENABLED_BIT,
FTRACE_EVENT_FL_RECORDED_CMD_BIT,
FTRACE_EVENT_FL_SOFT_MODE_BIT,
FTRACE_EVENT_FL_SOFT_DISABLED_BIT,
};
/*
* Ftrace event file flags:
* ENABLED - The event is enabled
* RECORDED_CMD - The comms should be recorded at sched_switch
* SOFT_MODE - The event is enabled/disabled by SOFT_DISABLED
* SOFT_DISABLED - When set, do not trace the event (even though its
* tracepoint may be enabled)
*/
enum {
FTRACE_EVENT_FL_ENABLED = (1 << FTRACE_EVENT_FL_ENABLED_BIT),
FTRACE_EVENT_FL_RECORDED_CMD = (1 << FTRACE_EVENT_FL_RECORDED_CMD_BIT),
FTRACE_EVENT_FL_SOFT_MODE = (1 << FTRACE_EVENT_FL_SOFT_MODE_BIT),
FTRACE_EVENT_FL_SOFT_DISABLED = (1 << FTRACE_EVENT_FL_SOFT_DISABLED_BIT),
};
struct ftrace_event_file {
@ -274,17 +281,18 @@ struct ftrace_event_file {
* 32 bit flags:
* bit 0: enabled
* bit 1: enabled cmd record
* bit 2: enable/disable with the soft disable bit
* bit 3: soft disabled
*
* Changes to flags must hold the event_mutex.
*
* Note: Reads of flags do not hold the event_mutex since
* they occur in critical sections. But the way flags
* Note: The bits must be set atomically to prevent races
* from other writers. Reads of flags do not need to be in
* sync as they occur in critical sections. But the way flags
* is currently used, these changes do not affect the code
* except that when a change is made, it may have a slight
* delay in propagating the changes to other CPUs due to
* caching and such.
* caching and such. Which is mostly OK ;-)
*/
unsigned int flags;
unsigned long flags;
};
#define __TRACE_EVENT_FLAGS(name, value) \