tracing/histogram: Update the documentation for the buckets modifier

Update both the tracefs README file as well as the histogram.rst to
include an explanation of what the buckets modifier is and how to use it.
Include an example with the wakeup_latency example for both log2 and the
buckets modifiers as there was no existing log2 example.

Link: https://lkml.kernel.org/r/20210707213922.167218794@goodmis.orgAcked-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/histogram.rst

@@ -77,6 +77,7 @@ Documentation written by Tom Zanussi
 	.syscall    display a syscall id as a system call name
 	.execname   display a common_pid as a program name
 	.log2       display log2 value rather than raw number
+	.buckets=size  display grouping of values rather than raw number
 	.usecs      display a common_timestamp in microseconds
 	=========== ==========================================
 
@@ -228,7 +229,7 @@ Extended error information
   that lists the total number of bytes requested for each function in
   the kernel that made one or more calls to kmalloc::
 
-    # echo 'hist:key=call_site:val=bytes_req' > \
+    # echo 'hist:key=call_site:val=bytes_req.buckets=32' > \
             /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
 
   This tells the tracing system to create a 'hist' trigger using the
@@ -1823,20 +1824,99 @@ and variables defined on other events (see Section 2.2.3 below on
 how that is done using hist trigger 'onmatch' action). Once that is
 done, the 'wakeup_latency' synthetic event instance is created.
 
-A histogram can now be defined for the new synthetic event::
-
-  # echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
-        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
-
 The new event is created under the tracing/events/synthetic/ directory
 and looks and behaves just like any other event::
 
   # ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
         enable  filter  format  hist  id  trigger
 
+A histogram can now be defined for the new synthetic event::
+
+  # echo 'hist:keys=pid,prio,lat.log2:sort=lat' >> \
+        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
+
+The above shows the latency "lat" in a power of 2 grouping.
+
 Like any other event, once a histogram is enabled for the event, the
 output can be displayed by reading the event's 'hist' file.
 
+  # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist
+
+  # event histogram
+  #
+  # trigger info: hist:keys=pid,prio,lat.log2:vals=hitcount:sort=lat.log2:size=2048 [active]
+  #
+
+  { pid:       2035, prio:          9, lat: ~ 2^2  } hitcount:         43
+  { pid:       2034, prio:          9, lat: ~ 2^2  } hitcount:         60
+  { pid:       2029, prio:          9, lat: ~ 2^2  } hitcount:        965
+  { pid:       2034, prio:        120, lat: ~ 2^2  } hitcount:          9
+  { pid:       2033, prio:        120, lat: ~ 2^2  } hitcount:          5
+  { pid:       2030, prio:          9, lat: ~ 2^2  } hitcount:        335
+  { pid:       2030, prio:        120, lat: ~ 2^2  } hitcount:         10
+  { pid:       2032, prio:        120, lat: ~ 2^2  } hitcount:          1
+  { pid:       2035, prio:        120, lat: ~ 2^2  } hitcount:          2
+  { pid:       2031, prio:          9, lat: ~ 2^2  } hitcount:        176
+  { pid:       2028, prio:        120, lat: ~ 2^2  } hitcount:         15
+  { pid:       2033, prio:          9, lat: ~ 2^2  } hitcount:         91
+  { pid:       2032, prio:          9, lat: ~ 2^2  } hitcount:        125
+  { pid:       2029, prio:        120, lat: ~ 2^2  } hitcount:          4
+  { pid:       2031, prio:        120, lat: ~ 2^2  } hitcount:          3
+  { pid:       2029, prio:        120, lat: ~ 2^3  } hitcount:          2
+  { pid:       2035, prio:          9, lat: ~ 2^3  } hitcount:         41
+  { pid:       2030, prio:        120, lat: ~ 2^3  } hitcount:          1
+  { pid:       2032, prio:          9, lat: ~ 2^3  } hitcount:         32
+  { pid:       2031, prio:          9, lat: ~ 2^3  } hitcount:         44
+  { pid:       2034, prio:          9, lat: ~ 2^3  } hitcount:         40
+  { pid:       2030, prio:          9, lat: ~ 2^3  } hitcount:         29
+  { pid:       2033, prio:          9, lat: ~ 2^3  } hitcount:         31
+  { pid:       2029, prio:          9, lat: ~ 2^3  } hitcount:         31
+  { pid:       2028, prio:        120, lat: ~ 2^3  } hitcount:         18
+  { pid:       2031, prio:        120, lat: ~ 2^3  } hitcount:          2
+  { pid:       2028, prio:        120, lat: ~ 2^4  } hitcount:          1
+  { pid:       2029, prio:          9, lat: ~ 2^4  } hitcount:          4
+  { pid:       2031, prio:        120, lat: ~ 2^7  } hitcount:          1
+  { pid:       2032, prio:        120, lat: ~ 2^7  } hitcount:          1
+
+  Totals:
+      Hits: 2122
+      Entries: 30
+      Dropped: 0
+
+
+The latency values can also be grouped linearly by a given size with
+the ".buckets" modifier and specify a size (in this case groups of 10).
+
+  # echo 'hist:keys=pid,prio,lat.buckets=10:sort=lat' >> \
+        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
+
+  # event histogram
+  #
+  # trigger info: hist:keys=pid,prio,lat.buckets=10:vals=hitcount:sort=lat.buckets=10:size=2048 [active]
+  #
+
+  { pid:       2067, prio:          9, lat: ~ 0-9 } hitcount:        220
+  { pid:       2068, prio:          9, lat: ~ 0-9 } hitcount:        157
+  { pid:       2070, prio:          9, lat: ~ 0-9 } hitcount:        100
+  { pid:       2067, prio:        120, lat: ~ 0-9 } hitcount:          6
+  { pid:       2065, prio:        120, lat: ~ 0-9 } hitcount:          2
+  { pid:       2066, prio:        120, lat: ~ 0-9 } hitcount:          2
+  { pid:       2069, prio:          9, lat: ~ 0-9 } hitcount:        122
+  { pid:       2069, prio:        120, lat: ~ 0-9 } hitcount:          8
+  { pid:       2070, prio:        120, lat: ~ 0-9 } hitcount:          1
+  { pid:       2068, prio:        120, lat: ~ 0-9 } hitcount:          7
+  { pid:       2066, prio:          9, lat: ~ 0-9 } hitcount:        365
+  { pid:       2064, prio:        120, lat: ~ 0-9 } hitcount:         35
+  { pid:       2065, prio:          9, lat: ~ 0-9 } hitcount:        998
+  { pid:       2071, prio:          9, lat: ~ 0-9 } hitcount:         85
+  { pid:       2065, prio:          9, lat: ~ 10-19 } hitcount:          2
+  { pid:       2064, prio:        120, lat: ~ 10-19 } hitcount:          2
+
+  Totals:
+      Hits: 2112
+      Entries: 16
+      Dropped: 0
+
 2.2.3 Hist trigger 'handlers' and 'actions'
 -------------------------------------------
 

kernel/trace/trace.c

@@ -5654,6 +5654,7 @@ static const char readme_msg[] =
 	"\t            .execname   display a common_pid as a program name\n"
 	"\t            .syscall    display a syscall id as a syscall name\n"
 	"\t            .log2       display log2 value rather than raw number\n"
+	"\t            .buckets=size  display values in groups of size rather than raw number\n"
 	"\t            .usecs      display a common_timestamp in microseconds\n\n"
 	"\t    The 'pause' parameter can be used to pause an existing hist\n"
 	"\t    trigger or to start a hist trigger but not log any events\n"