[PATCH 1/2] add-tracing_max_latency-may-not-be-worst-case-system-latency-to-ftrace-docs.patch

From: Carsten Emde
Date: Sat Dec 26 2009 - 17:11:16 EST


Explain the background of the trace variable
tracing_max_latency in the context of the
wakeup and the wakeup_rt tracer and why it
does not necessarily measure the worst-case
latency of the system.

Signed-off-by: Carsten Emde <C.Emde@xxxxxxxxx>

Index: linux-2.6.33-rc2/Documentation/trace/ftrace.txt
===================================================================
--- linux-2.6.33-rc2.orig/Documentation/trace/ftrace.txt
+++ linux-2.6.33-rc2/Documentation/trace/ftrace.txt
@@ -117,9 +117,25 @@ of ftrace. Here is a list of some of the
For example, the time interrupts are disabled.
This time is saved in this file. The max trace
will also be stored, and displayed by "trace".
- A new max trace will only be recorded if the
+ A new max trace will only be recorded, if the
latency is greater than the value in this
- file. (in microseconds)
+ file (in microseconds). NOTE: The max latency
+ recorded by the wakeup and the wakeup_rt tracer
+ do not necessarily reflect the worst-case latency
+ of the system. If two or more processes share the
+ highest priority of the system and may be woken
+ up shortly after each other, the max latency
+ recorded by the wakeup and the wakeup_rt tracers
+ equal the worst-case latency of the system *plus*
+ the longest possible runtime duration of all tasks
+ that share the same priority. An exact measurement
+ of the worst-case wakeup latency of the system can
+ be achieved using the wakekup latency histograms
+ that are part of the PREEMPT_RT patches. These
+ histograms separately determine the wakeup latency
+ of a task that exclusively uses the highest
+ priority of the system and of tasks that share the
+ highest priority of the system with other tasks.

buffer_size_kb:


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