commit bf287607c8 upstream.
It turned out we used to use default implementation of sched_clock()
from kernel/sched/clock.c which was as precise as 1/HZ, i.e.
by default we had 10 msec granularity of time measurement.
Now given ARC built-in timers are clocked with the same frequency as
CPU cores we may get much higher precision of time tracking.
Thus we switch to generic sched_clock which really reads ARC hardware
counters.
This is especially helpful for measuring short events.
That's what we used to have:
------------------------------>8------------------------
$ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null
Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':
10.000000 task-clock (msec) # 2.832 CPUs utilized
1 context-switches # 0.100 K/sec
1 cpu-migrations # 0.100 K/sec
63 page-faults # 0.006 M/sec
3049480 cycles # 0.305 GHz
1091259 instructions # 0.36 insn per cycle
256828 branches # 25.683 M/sec
27026 branch-misses # 10.52% of all branches
0.003530687 seconds time elapsed
0.000000000 seconds user
0.010000000 seconds sys
------------------------------>8------------------------
And now we'll see:
------------------------------>8------------------------
$ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null
Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':
3.004322 task-clock (msec) # 0.865 CPUs utilized
1 context-switches # 0.333 K/sec
1 cpu-migrations # 0.333 K/sec
63 page-faults # 0.021 M/sec
2986734 cycles # 0.994 GHz
1087466 instructions # 0.36 insn per cycle
255209 branches # 84.947 M/sec
26002 branch-misses # 10.19% of all branches
0.003474829 seconds time elapsed
0.003519000 seconds user
0.000000000 seconds sys
------------------------------>8------------------------
Note how much more meaningful is the second output - time spent for
execution pretty much matches number of cycles spent (we're runnign
@ 1GHz here).
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
bf75d9382b