Update readme: discuss perf counters

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/README b/README
index 583d0e024864d2e37fd22a86d847ec955539b1d7..1a7cea16e4ca49e72ae957e1d3723edb8d746a19 100644 (file)
--- a/README
+++ b/README
@@ -46,3 +46,10 @@ Support for LTTng 0.x clocks will be added back soon into LTTng 2.0.  Please
 note that lttng-modules 2.0 can build on a Linux kernel patched with the LTTng
 0.x patchset, but the lttng-modules 2.0 replace the lttng-modules 0.x, so both
 tracers cannot be installed at the same time for a given kernel version.
+
+* Note about Perf PMU counters support
+
+Each PMU counter has its zero value set when it is attached to a context with
+add-context. Therefore, it is normal that the same counters attached to both the
+stream context and event context show different values for a given event; what
+matters is that they increment at the same rate.