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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: 5185432
{ 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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.