Example output:
# echo 1> /debug/tracing/events/readahead/enable
# cp test-file /dev/null
# cat /debug/tracing/trace # trimmed output
readahead-initial(dev=0:15, ino=100177, req=0+2, ra=0+4-2, async=0) = 4
readahead-subsequent(dev=0:15, ino=100177, req=2+2, ra=4+8-8, async=1) = 8
readahead-subsequent(dev=0:15, ino=100177, req=4+2, ra=12+16-16, async=1) = 16
readahead-subsequent(dev=0:15, ino=100177, req=12+2, ra=28+32-32, async=1) = 32
readahead-subsequent(dev=0:15, ino=100177, req=28+2, ra=60+60-60, async=1) = 24
readahead-subsequent(dev=0:15, ino=100177, req=60+2, ra=120+60-60, async=1) = 0
CC: Ingo Molnar<mingo@xxxxxxx>
CC: Jens Axboe<jens.axboe@xxxxxxxxxx>
CC: Steven Rostedt<rostedt@xxxxxxxxxxx>
CC: Peter Zijlstra<a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Wu Fengguang<fengguang.wu@xxxxxxxxx>