ftrace: Document filter and option limitations
Subsystems (3)
PATCH v2 1/1 ftrace: Document filter and option limitations
When using the function or function_graph tracers from the command line, certain command line options have limitations.
Document that only kernel built-in functions can be filtered via ftrace_filter= or ftrace_graph_filter=. Document that tracer-specific options cannot be set on the command line via trace_options. Document that ftrace cannot do late binding for function filters.
--- Documentation/kernel-parameters.txt | 6 ++++++ Documentation/trace/ftrace.txt | 7 +++++-- 2 files changed, 11 insertions(+), 2 deletions(-)
Documentation/kernel-parameters.txt +6 -0
@@ -1121,6 +1121,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted. list of functions. This list can be changed at run time by the set_ftrace_filter file in the debugfs tracing directory. + Only kernel built-in functions can be filtered. ftrace_notrace=[function-list] [FTRACE] Do not trace the functions specified in
@@ -1134,6 +1135,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted. function-list is a comma separated list of functions that can be changed at run time by the set_graph_function file in the debugfs tracing directory. + Only kernel built-in functions can be filtered. ftrace_graph_notrace=[function-list] [FTRACE] Do not trace from the functions specified in
@@ -3521,6 +3523,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted. trace_options=stacktrace + Tracer-specific options are ignored when set this way. + For example, the 'func_stack_trace' option cannot be + set here. + See also Documentation/trace/ftrace.txt "trace options" section.
Documentation/trace/ftrace.txt +5 -2
@@ -188,7 +188,8 @@ of ftrace. Here is a list of some of the key files: in with practically no overhead in performance. This also has a side effect of enabling or disabling specific functions to be traced. Echoing names of functions into this file - will limit the trace to only those functions. + will limit the trace to only those functions. Only already- + loaded functions can be filtered. This interface also allows for commands to be used. See the "Filter commands" section for more details.
@@ -208,7 +209,9 @@ of ftrace. Here is a list of some of the key files: Set a "trigger" function where tracing should start with the function graph tracer (See the section - "dynamic ftrace" for more details). + "dynamic ftrace" for more details). As with set_ftrace_filter, + only names of already-loaded functions can be written to + this file. available_filter_functions:
Signature · 1 line
2.1.1
Steven Rostedt
maintainerQuoted 51 lines · On Tue, 4 Nov 2014 12:46:05 -0500 Peter Hurley <[email protected]>…
When using the function or
function_graphtracers from the command line, certain command line options have limitations.Document that only kernel built-in functions can be filtered via ftrace_filter= or ftrace_graph_filter=. Document that tracer-specific options cannot be set on the command line via
trace_options. Document that ftrace cannot do late binding for function filters.Signed-off-by Peter Hurley <[email protected]>--- Documentation/kernel-parameters.txt | 6 ++++++ Documentation/trace/ftrace.txt | 7 +++++-- 2 files changed, 11 insertions(+), 2 deletions(-)Documentation/kernel-parameters.txt +6
-0@@ -1121,6 +1121,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted. list of functions. This list can be changed at run time by the set_ftrace_filter file in the debugfs tracing directory. + Only kernel built-in functions can be filtered. ftrace_notrace=[function-list] [FTRACE] Do not trace the functions specified in@@ -1134,6 +1135,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted. function-list is a comma separated list of functions that can be changed at run time by the set_graph_function file in the debugfs tracing directory. + Only kernel built-in functions can be filtered. ftrace_graph_notrace=[function-list] [FTRACE] Do not trace from the functions specified in@@ -3521,6 +3523,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted. trace_options=stacktrace + Tracer-specific options are ignored when set this way. + For example, the 'func_stack_trace' option cannot be + set here. + See also Documentation/trace/ftrace.txt "trace options" section.Documentation/trace/ftrace.txt +0
-0
Documentation/trace/ftrace.txt of ftrace. Here is a list of some of the key files: · line 192
Quoted 6 lines
Documentation/trace/ftrace.txt +2
-1@@ -188,7 +188,8 @@ of ftrace. Here is a list of some of the key files: in with practically no overhead in performance. This also has a side effect of enabling or disabling specific functions to be traced. Echoing names of functions into this file - will limit the trace to only those functions. + will limit the trace to only those functions. Only already- + loaded functions can be filtered.
I would add a little more. Something that states that modules with the same function names that are loaded at a later time will not be filtered with theses names.
Hmm, actually, I think this could be rather trivial to implement something that will filter modules that are added.
I think that may be a better solution. I'm assuming you would like to have functions filtered via the command line to filter on modules as well. Correct?
-- Steve
Quoted 14 lines
Documentation/trace/ftrace.txt +3
-1This interface also allows for commands to be used. See the "Filter commands" section for more details.@@ -208,7 +209,9 @@ of ftrace. Here is a list of some of the key files: Set a "trigger" function where tracing should start with the function graph tracer (See the section - "dynamic ftrace" for more details). + "dynamic ftrace" for more details). As with set_ftrace_filter, + only names of already-loaded functions can be written to + this file. available_filter_functions:
Quoted 49 lines · Steven Rostedt
Quoted 46 lines · On Tue, 4 Nov 2014 12:46:05 -0500 Peter Hurley <[email protected]>…
When using the function or
function_graphtracers from the command line, certain command line options have limitations.Document that only kernel built-in functions can be filtered via ftrace_filter= or ftrace_graph_filter=. Document that tracer-specific options cannot be set on the command line via
trace_options. Document that ftrace cannot do late binding for function filters.Signed-off-by Peter Hurley <[email protected]>--- Documentation/kernel-parameters.txt | 6 ++++++ Documentation/trace/ftrace.txt | 7 +++++-- 2 files changed, 11 insertions(+), 2 deletions(-)Documentation/kernel-parameters.txt +6
-0@@ -1121,6 +1121,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted. list of functions. This list can be changed at run time by the set_ftrace_filter file in the debugfs tracing directory. + Only kernel built-in functions can be filtered. ftrace_notrace=[function-list] [FTRACE] Do not trace the functions specified in@@ -1134,6 +1135,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted. function-list is a comma separated list of functions that can be changed at run time by the set_graph_function file in the debugfs tracing directory. + Only kernel built-in functions can be filtered. ftrace_graph_notrace=[function-list] [FTRACE] Do not trace from the functions specified in@@ -3521,6 +3523,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted. trace_options=stacktrace + Tracer-specific options are ignored when set this way. + For example, the 'func_stack_trace' option cannot be + set here. + See also Documentation/trace/ftrace.txt "trace options" section.
Quoted 15 lines
Documentation/trace/ftrace.txt of ftrace. Here is a list of some of the key files: · line 192
Quoted 11 lines
Documentation/trace/ftrace.txt +2
-1@@ -188,7 +188,8 @@ of ftrace. Here is a list of some of the key files: in with practically no overhead in performance. This also has a side effect of enabling or disabling specific functions to be traced. Echoing names of functions into this file - will limit the trace to only those functions. + will limit the trace to only those functions. Only already- + loaded functions can be filtered.I would add a little more. Something that states that modules with the same function names that are loaded at a later time will not be filtered with theses names.
I could add that trying to write function names from non-existent or not loaded modules returns EINVAL.
Quoted 6 lines
Hmm, actually, I think this could be rather trivial to implement something that will filter modules that are added.
I think that may be a better solution. I'm assuming you would like to have functions filtered via the command line to filter on modules as well. Correct?
I think late binding would be a nice feature to add even if not possible from the kernel command line. I definitely could have used that a year ago or so when I was debugging firewire subsystem races on module load.
The command line stuff came up because I was trying to trace drm mode-setting, but once I knew why filtering wasn't working, it was trivial to just build in drm. So I'm not sure it's really worth it to make the command line options do the same thing, but I think documenting it is important to save people time looking into why it doesn't work.
Regards,
Peter Hurley