tracing: Disable tracing on warning

Add a traceoff_on_warning option in both the kernel command line as well
as a sysctl option. When set, any WARN*() function that is hit will cause
the tracing_on variable to be cleared, which disables writing to the
ring buffer.

This is useful especially when tracing a bug with function tracing. When
a warning is hit, the print caused by the warning can flood the trace with
the functions that producing the output for the warning. This can make the
resulting trace useless by either hiding where the bug happened, or worse,
by overflowing the buffer and losing the trace of the bug totally.

Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 6e3b18a8afc6..729d0b9803b7 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3069,6 +3069,19 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         See also Documentation/trace/ftrace.txt "trace options"
                         section.
 
+        traceoff_on_warning
+                        [FTRACE] enable this option to disable tracing when a
+                        warning is hit. This turns off "tracing_on". Tracing can
+                        be enabled again by echoing '1' into the "tracing_on"
+                        file located in /sys/kernel/debug/tracing/
+
+                        This option is useful, as it disables the trace before
+                        the WARNING dump is called, which prevents the trace to
+                        be filled with content caused by the warning output.
+
+                        This option can also be set at run time via the sysctl
+                        option:  kernel/traceoff_on_warning
+
         transparent_hugepage=
                         [KNL]
                         Format: [always|madvise|never]