Due to retpolines, indirect calls are much more expensive than direct
calls. The histograms have a select set of functions it uses for the
histograms, instead of using function pointers to call them, create a
hist_fn_call() function that uses a switch statement to call the histogram
functions directly. This gives a 13% speedup to the histogram logic.

Using the histogram benchmark:

Before:

 # event histogram
 #
 # trigger info: hist:keys=delta:vals=hitcount:sort=delta:size=2048 if delta > 0 [active]
 #

{ delta:        129 } hitcount:       2213
{ delta:        130 } hitcount:     285965
{ delta:        131 } hitcount:    1146545
{ delta:        132 } hitcount:    51854322
{ delta:        133 } hitcount:   19896215
{ delta:        134 } hitcount:   53118616
{ delta:        135 } hitcount:   83816709
{ delta:        136 } hitcount:   68329562
{ delta:        137 } hitcount:   41859349
{ delta:        138 } hitcount:   46257797
{ delta:        139 } hitcount:   54400831
{ delta:        140 } hitcount:   72875007
{ delta:        141 } hitcount:   76193272
{ delta:        142 } hitcount:   49504263
{ delta:        143 } hitcount:   38821072
{ delta:        144 } hitcount:   47702679
{ delta:        145 } hitcount:   41357297
{ delta:        146 } hitcount:   22058238
{ delta:        147 } hitcount:    9720002
{ delta:        148 } hitcount:    3193542
{ delta:        149 } hitcount:     927030
{ delta:        150 } hitcount:     850772
{ delta:        151 } hitcount:    1477380
{ delta:        152 } hitcount:    2687977
{ delta:        153 } hitcount:    2865985
{ delta:        154 } hitcount:    1977492
{ delta:        155 } hitcount:    2475607
{ delta:        156 } hitcount:    3403612

After:

 # event histogram
 #
 # trigger info: hist:keys=delta:vals=hitcount:sort=delta:size=2048 if delta > 0 [active]
 #

{ delta:        113 } hitcount:        272
{ delta:        114 } hitcount:        840
{ delta:        118 } hitcount:        344
{ delta:        119 } hitcount:      25428
{ delta:        120 } hitcount:     350590
{ delta:        121 } hitcount:    1892484
{ delta:        122 } hitcount:    6205004
{ delta:        123 } hitcount:   11583521
{ delta:        124 } hitcount:   37590979
{ delta:        125 } hitcount:  108308504
{ delta:        126 } hitcount:  131672461
{ delta:        127 } hitcount:   88700598
{ delta:        128 } hitcount:   65939870
{ delta:        129 } hitcount:   45055004
{ delta:        130 } hitcount:   33174464
{ delta:        131 } hitcount:   31813493
{ delta:        132 } hitcount:   29011676
{ delta:        133 } hitcount:   22798782
{ delta:        134 } hitcount:   22072486
{ delta:        135 } hitcount:   17034113
{ delta:        136 } hitcount:    8982490
{ delta:        137 } hitcount:    2865908
{ delta:        138 } hitcount:     980382
{ delta:        139 } hitcount:    1651944
{ delta:        140 } hitcount:    4112073
{ delta:        141 } hitcount:    3963269
{ delta:        142 } hitcount:    1712508
{ delta:        143 } hitcount:     575941
{ delta:        144 } hitcount:     351427
{ delta:        145 } hitcount:     218077
{ delta:        146 } hitcount:     167297
{ delta:        147 } hitcount:     146198
{ delta:        148 } hitcount:     116122
{ delta:        149 } hitcount:      58993
{ delta:        150 } hitcount:      40228

The delta above is in nanoseconds. It brings the fastest time down from
129ns to 113ns, and the peak from 141ns to 126ns.

Link: https://lkml.kernel.org/r/20220906225529.411545333@goodmis.org

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>