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authorTom Zanussi <tom.zanussi@linux.intel.com>2018-01-15 20:52:08 -0600
committerSteven Rostedt (VMware) <rostedt@goodmis.org>2018-03-10 16:06:03 -0500
commita4072fe85ba3671720cab0788291af953db27318 (patch)
treec96d124bab2370cb3ddb70bb8fe6f130c9a609ab /Documentation/trace
parentd71bd34d78bb78b9e6f8a0be3952d5fa470a260a (diff)
downloadlinux-a4072fe85ba3671720cab0788291af953db27318.tar.bz2
tracing: Add a clock attribute for hist triggers
The default clock if timestamps are used in a histogram is "global". If timestamps aren't used, the clock is irrelevant. Use the "clock=" param only if you want to override the default "global" clock for a histogram with timestamps. Link: http://lkml.kernel.org/r/427bed1389c5d22aa40c3e0683e30cc3d151e260.1516069914.git.tom.zanussi@linux.intel.com Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com> Signed-off-by: Rajvi Jingar <rajvi.jingar@intel.com> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Diffstat (limited to 'Documentation/trace')
-rw-r--r--Documentation/trace/histogram.txt11
1 files changed, 10 insertions, 1 deletions
diff --git a/Documentation/trace/histogram.txt b/Documentation/trace/histogram.txt
index df08882d091c..6e05510afc28 100644
--- a/Documentation/trace/histogram.txt
+++ b/Documentation/trace/histogram.txt
@@ -1671,7 +1671,16 @@ features have been added to the hist trigger support:
it is in units of nanoseconds; appending '.usecs' to a
common_timestamp field changes the units to microseconds.
-These features are decribed in more detail in the following sections.
+A note on inter-event timestamps: If common_timestamp is used in a
+histogram, the trace buffer is automatically switched over to using
+absolute timestamps and the "global" trace clock, in order to avoid
+bogus timestamp differences with other clocks that aren't coherent
+across CPUs. This can be overridden by specifying one of the other
+trace clocks instead, using the "clock=XXX" hist trigger attribute,
+where XXX is any of the clocks listed in the tracing/trace_clock
+pseudo-file.
+
+These features are described in more detail in the following sections.
2.2.1 Histogram Variables
-------------------------