bus: arm-ccn: cpumask attribute

This patch adds a "cpumask" attribute to CCN's event_source class sysfs
directory. Perf user tool uses it to restrict events to the
processor(s) enumerated in this mask.

This patch provides a single CPU mask, making it possible to run "-a"
perf session (previously it would request the same CCN event, for
example cycle counter, on each available core and most likely fail).
Initially the mask is set to the CPU that happened to probe the driver,
but it will be changed when it is hot-un-plugged (active events are
migrated to another CPU then).

Example:

 Performance counter stats for 'system wide':

CPU0               2968148      cycles
CPU1               2236736      cycles
CPU2               1797968      cycles
CPU3               1831715      cycles
CPU1            1201850868      ccn/cycles/

       1.001241383 seconds time elapsed

Signed-off-by: Pawel Moll <pawel.moll@arm.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/Documentation/arm/CCN.txt b/Documentation/arm/CCN.txt
index 0632b3aad83e..3a7604874762 100644
--- a/Documentation/arm/CCN.txt
+++ b/Documentation/arm/CCN.txt
@@ -33,6 +33,13 @@ directory, with first 8 configurable by user and additional
 Cycle counter is described by a "type" value 0xff and does
 not require any other settings.
 
+The driver also provides a "cpumask" sysfs attribute, which contains
+a single CPU ID, of the processor which will be used to handle all
+the CCN PMU events. It is recommended that the user space tools
+request the events on this processor (if not, the perf_event->cpu value
+will be overwritten anyway). In case of this processor being offlined,
+the events are migrated to another one and the attribute is updated.
+
 Example of perf tool use:
 
 / # perf list | grep ccn
@@ -41,12 +48,8 @@ Example of perf tool use:
   ccn/xp_valid_flit/                                 [Kernel PMU event]
 <...>
 
-/ # perf stat -C 0 -e ccn/cycles/,ccn/xp_valid_flit,xp=1,port=0,vc=1,dir=1/ \
+/ # perf stat -a -e ccn/cycles/,ccn/xp_valid_flit,xp=1,port=0,vc=1,dir=1/ \
                                                                        sleep 1
 
 The driver does not support sampling, therefore "perf record" will
-not work. Also notice that only single cpu is being selected
-("-C 0") - this is because perf framework does not support
-"non-CPU related" counters (yet?) so system-wide session ("-a")
-would try (and in most cases fail) to set up the same event
-per each CPU.
+not work. Per-task (without "-a") perf sessions are not supported.