samples/ftrace/ftrace-ops.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2
   3 #define pr_fmt(fmt)       KBUILD_MODNAME ": " fmt
   4
   5 #include <linux/ftrace.h>
   6 #include <linux/ktime.h>
   7 #include <linux/module.h>
   8
   9 #include <asm/barrier.h>
  10
  11 /*
  12  * Arbitrary large value chosen to be sufficiently large to minimize noise but
  13  * sufficiently small to complete quickly.
  14  */
  15 static unsigned int nr_function_calls = 100000;
  16 module_param(nr_function_calls, uint, 0);
  17 MODULE_PARM_DESC(nr_function_calls, "How many times to call the relevant tracee");
  18
  19 /*
  20  * The number of ops associated with a call site affects whether a tracer can
  21  * be called directly or whether it's necessary to go via the list func, which
  22  * can be significantly more expensive.
  23  */
  24 static unsigned int nr_ops_relevant = 1;
  25 module_param(nr_ops_relevant, uint, 0);
  26 MODULE_PARM_DESC(nr_ops_relevant, "How many ftrace_ops to associate with the relevant tracee");
  27
  28 /*
  29  * On architectures where all call sites share the same trampoline, having
  30  * tracers enabled for distinct functions can force the use of the list func
  31  * and incur overhead for all call sites.
  32  */
  33 static unsigned int nr_ops_irrelevant;
  34 module_param(nr_ops_irrelevant, uint, 0);
  35 MODULE_PARM_DESC(nr_ops_irrelevant, "How many ftrace_ops to associate with the irrelevant tracee");
  36
  37 /*
  38  * On architectures with DYNAMIC_FTRACE_WITH_REGS, saving the full pt_regs can
  39  * be more expensive than only saving the minimal necessary regs.
  40  */
  41 static bool save_regs;
  42 module_param(save_regs, bool, 0);
  43 MODULE_PARM_DESC(save_regs, "Register ops with FTRACE_OPS_FL_SAVE_REGS (save all registers in the trampoline)");
  44
  45 static bool assist_recursion;
  46 module_param(assist_recursion, bool, 0);
  47 MODULE_PARM_DESC(assist_reursion, "Register ops with FTRACE_OPS_FL_RECURSION");
  48
  49 static bool assist_rcu;
  50 module_param(assist_rcu, bool, 0);
  51 MODULE_PARM_DESC(assist_reursion, "Register ops with FTRACE_OPS_FL_RCU");
  52
  53 /*
  54  * By default, a trivial tracer is used which immediately returns to mimimize
  55  * overhead. Sometimes a consistency check using a more expensive tracer is
  56  * desireable.
  57  */
  58 static bool check_count;
  59 module_param(check_count, bool, 0);
  60 MODULE_PARM_DESC(check_count, "Check that tracers are called the expected number of times\n");
  61
  62 /*
  63  * Usually it's not interesting to leave the ops registered after the test
  64  * runs, but sometimes it can be useful to leave them registered so that they
  65  * can be inspected through the tracefs 'enabled_functions' file.
  66  */
  67 static bool persist;
  68 module_param(persist, bool, 0);
  69 MODULE_PARM_DESC(persist, "Successfully load module and leave ftrace ops registered after test completes\n");
  70
  71 /*
  72  * Marked as noinline to ensure that an out-of-line traceable copy is
  73  * generated by the compiler.
  74  *
  75  * The barrier() ensures the compiler won't elide calls by determining there
  76  * are no side-effects.
  77  */
  78 static noinline void tracee_relevant(void)
  79 {
  80         barrier();
  81 }
  82
  83 /*
  84  * Marked as noinline to ensure that an out-of-line traceable copy is
  85  * generated by the compiler.
  86  *
  87  * The barrier() ensures the compiler won't elide calls by determining there
  88  * are no side-effects.
  89  */
  90 static noinline void tracee_irrelevant(void)
  91 {
  92         barrier();
  93 }
  94
  95 struct sample_ops {
  96         struct ftrace_ops ops;
  97         unsigned int count;
  98 };
  99
 100 static void ops_func_nop(unsigned long ip, unsigned long parent_ip,
 101                          struct ftrace_ops *op,
 102                          struct ftrace_regs *fregs)
 103 {
 104         /* do nothing */
 105 }
 106
 107 static void ops_func_count(unsigned long ip, unsigned long parent_ip,
 108                            struct ftrace_ops *op,
 109                            struct ftrace_regs *fregs)
 110 {
 111         struct sample_ops *self;
 112
 113         self = container_of(op, struct sample_ops, ops);
 114         self->count++;
 115 }
 116
 117 static struct sample_ops *ops_relevant;
 118 static struct sample_ops *ops_irrelevant;
 119
 120 static struct sample_ops *ops_alloc_init(void *tracee, ftrace_func_t func,
 121                                          unsigned long flags, int nr)
 122 {
 123         struct sample_ops *ops;
 124
 125         ops = kcalloc(nr, sizeof(*ops), GFP_KERNEL);
 126         if (WARN_ON_ONCE(!ops))
 127                 return NULL;
 128
 129         for (unsigned int i = 0; i < nr; i++) {
 130                 ops[i].ops.func = func;
 131                 ops[i].ops.flags = flags;
 132                 WARN_ON_ONCE(ftrace_set_filter_ip(&ops[i].ops, (unsigned long)tracee, 0, 0));
 133                 WARN_ON_ONCE(register_ftrace_function(&ops[i].ops));
 134         }
 135
 136         return ops;
 137 }
 138
 139 static void ops_destroy(struct sample_ops *ops, int nr)
 140 {
 141         if (!ops)
 142                 return;
 143
 144         for (unsigned int i = 0; i < nr; i++) {
 145                 WARN_ON_ONCE(unregister_ftrace_function(&ops[i].ops));
 146                 ftrace_free_filter(&ops[i].ops);
 147         }
 148
 149         kfree(ops);
 150 }
 151
 152 static void ops_check(struct sample_ops *ops, int nr,
 153                       unsigned int expected_count)
 154 {
 155         if (!ops || !check_count)
 156                 return;
 157
 158         for (unsigned int i = 0; i < nr; i++) {
 159                 if (ops->count == expected_count)
 160                         continue;
 161                 pr_warn("Counter called %u times (expected %u)\n",
 162                         ops->count, expected_count);
 163         }
 164 }
 165
 166 static ftrace_func_t tracer_relevant = ops_func_nop;
 167 static ftrace_func_t tracer_irrelevant = ops_func_nop;
 168
 169 static int __init ftrace_ops_sample_init(void)
 170 {
 171         unsigned long flags = 0;
 172         ktime_t start, end;
 173         u64 period;
 174
 175         if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) && save_regs) {
 176                 pr_info("this kernel does not support saving registers\n");
 177                 save_regs = false;
 178         } else if (save_regs) {
 179                 flags |= FTRACE_OPS_FL_SAVE_REGS;
 180         }
 181
 182         if (assist_recursion)
 183                 flags |= FTRACE_OPS_FL_RECURSION;
 184
 185         if (assist_rcu)
 186                 flags |= FTRACE_OPS_FL_RCU;
 187
 188         if (check_count) {
 189                 tracer_relevant = ops_func_count;
 190                 tracer_irrelevant = ops_func_count;
 191         }
 192
 193         pr_info("registering:\n"
 194                 "  relevant ops: %u\n"
 195                 "    tracee: %ps\n"
 196                 "    tracer: %ps\n"
 197                 "  irrelevant ops: %u\n"
 198                 "    tracee: %ps\n"
 199                 "    tracer: %ps\n"
 200                 "  saving registers: %s\n"
 201                 "  assist recursion: %s\n"
 202                 "  assist RCU: %s\n",
 203                 nr_ops_relevant, tracee_relevant, tracer_relevant,
 204                 nr_ops_irrelevant, tracee_irrelevant, tracer_irrelevant,
 205                 save_regs ? "YES" : "NO",
 206                 assist_recursion ? "YES" : "NO",
 207                 assist_rcu ? "YES" : "NO");
 208
 209         ops_relevant = ops_alloc_init(tracee_relevant, tracer_relevant,
 210                                       flags, nr_ops_relevant);
 211         ops_irrelevant = ops_alloc_init(tracee_irrelevant, tracer_irrelevant,
 212                                         flags, nr_ops_irrelevant);
 213
 214         start = ktime_get();
 215         for (unsigned int i = 0; i < nr_function_calls; i++)
 216                 tracee_relevant();
 217         end = ktime_get();
 218
 219         ops_check(ops_relevant, nr_ops_relevant, nr_function_calls);
 220         ops_check(ops_irrelevant, nr_ops_irrelevant, 0);
 221
 222         period = ktime_to_ns(ktime_sub(end, start));
 223
 224         pr_info("Attempted %u calls to %ps in %lluns (%lluns / call)\n",
 225                 nr_function_calls, tracee_relevant,
 226                 period, div_u64(period, nr_function_calls));
 227
 228         if (persist)
 229                 return 0;
 230
 231         ops_destroy(ops_relevant, nr_ops_relevant);
 232         ops_destroy(ops_irrelevant, nr_ops_irrelevant);
 233
 234         /*
 235          * The benchmark completed sucessfully, but there's no reason to keep
 236          * the module around. Return an error do the user doesn't have to
 237          * manually unload the module.
 238          */
 239         return -EINVAL;
 240 }
 241 module_init(ftrace_ops_sample_init);
 242
 243 static void __exit ftrace_ops_sample_exit(void)
 244 {
 245         ops_destroy(ops_relevant, nr_ops_relevant);
 246         ops_destroy(ops_irrelevant, nr_ops_irrelevant);
 247 }
 248 module_exit(ftrace_ops_sample_exit);
 249
 250 MODULE_AUTHOR("Mark Rutland");
 251 MODULE_DESCRIPTION("Example of using custom ftrace_ops");
 252 MODULE_LICENSE("GPL");