wrapper/trace-clock.h

   1 /* SPDX-License-Identifier: (GPL-2.0-only or LGPL-2.1-only)
   2  *
   3  * wrapper/trace-clock.h
   4  *
   5  * Contains LTTng trace clock mapping to LTTng trace clock or mainline monotonic
   6  * clock. This wrapper depends on CONFIG_HIGH_RES_TIMERS=y.
   7  *
   8  * Copyright (C) 2011-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   9  */
  10
  11 #ifndef _LTTNG_TRACE_CLOCK_H
  12 #define _LTTNG_TRACE_CLOCK_H
  13
  14 #ifdef CONFIG_HAVE_TRACE_CLOCK
  15 #include <linux/trace-clock.h>
  16 #else /* CONFIG_HAVE_TRACE_CLOCK */
  17
  18 #include <linux/hardirq.h>
  19 #include <linux/ktime.h>
  20 #include <linux/time.h>
  21 #include <linux/hrtimer.h>
  22 #include <linux/percpu.h>
  23 #include <linux/version.h>
  24 #include <linux/percpu-defs.h>
  25 #include <asm/local.h>
  26 #include <lttng-kernel-version.h>
  27 #include <lttng-clock.h>
  28 #include <linux/random.h>
  29
  30 extern struct lttng_trace_clock *lttng_trace_clock;
  31
  32 /*
  33  * We need clock values to be monotonically increasing per-cpu, which is
  34  * not strictly guaranteed by ktime_get_mono_fast_ns(). It is
  35  * straightforward to do on architectures with a 64-bit cmpxchg(), but
  36  * not so on architectures without 64-bit cmpxchg. For now, only enable
  37  * this feature on 64-bit architectures.
  38  */
  39
  40 #if BITS_PER_LONG == 64
  41 #define LTTNG_USE_NMI_SAFE_CLOCK
  42 #endif
  43
  44 #ifdef LTTNG_USE_NMI_SAFE_CLOCK
  45
  46 DECLARE_PER_CPU(u64, lttng_last_tsc);
  47
  48 /*
  49  * Sometimes called with preemption enabled. Can be interrupted.
  50  */
  51 static inline u64 trace_clock_monotonic_wrapper(void)
  52 {
  53         u64 now, last, result;
  54         u64 *last_tsc_ptr;
  55
  56         /* Use fast nmi-safe monotonic clock provided by the Linux kernel. */
  57         preempt_disable();
  58         last_tsc_ptr = this_cpu_ptr(&lttng_last_tsc);
  59         last = *last_tsc_ptr;
  60         /*
  61          * Read "last" before "now". It is not strictly required, but it ensures
  62          * that an interrupt coming in won't artificially trigger a case where
  63          * "now" < "last". This kind of situation should only happen if the
  64          * mono_fast time source goes slightly backwards.
  65          */
  66         barrier();
  67         now = ktime_get_mono_fast_ns();
  68         if (U64_MAX / 2 < now - last)
  69                 now = last;
  70         result = cmpxchg64_local(last_tsc_ptr, last, now);
  71         preempt_enable();
  72         if (result == last) {
  73                 /* Update done. */
  74                 return now;
  75         } else {
  76                 /*
  77                  * Update not done, due to concurrent update. We can use
  78                  * "result", since it has been sampled concurrently with our
  79                  * time read, so it should not be far from "now".
  80                  */
  81                 return result;
  82         }
  83 }
  84
  85 #else /* #ifdef LTTNG_USE_NMI_SAFE_CLOCK */
  86 static inline u64 trace_clock_monotonic_wrapper(void)
  87 {
  88         ktime_t ktime;
  89
  90         /*
  91          * Refuse to trace from NMIs with this wrapper, because an NMI could
  92          * nest over the xtime write seqlock and deadlock.
  93          */
  94         if (in_nmi())
  95                 return (u64) -EIO;
  96
  97         ktime = ktime_get();
  98         return ktime_to_ns(ktime);
  99 }
 100 #endif /* #else #ifdef LTTNG_USE_NMI_SAFE_CLOCK */
 101
 102 static inline u64 trace_clock_read64_monotonic(void)
 103 {
 104         return (u64) trace_clock_monotonic_wrapper();
 105 }
 106
 107 static inline u64 trace_clock_freq_monotonic(void)
 108 {
 109         return (u64) NSEC_PER_SEC;
 110 }
 111
 112 static inline int trace_clock_uuid_monotonic(char *uuid)
 113 {
 114         unsigned char *boot_id;
 115
 116         boot_id = get_kernel_boot_id();
 117         sprintf(uuid, "%pU", boot_id);
 118         return 0;
 119 }
 120
 121 static inline const char *trace_clock_name_monotonic(void)
 122 {
 123         return "monotonic";
 124 }
 125
 126 static inline const char *trace_clock_description_monotonic(void)
 127 {
 128         return "Monotonic Clock";
 129 }
 130
 131 #ifdef LTTNG_USE_NMI_SAFE_CLOCK
 132 static inline int get_trace_clock(void)
 133 {
 134         printk_once(KERN_WARNING "LTTng: Using mainline kernel monotonic fast clock, which is NMI-safe.\n");
 135         return 0;
 136 }
 137 #else /* #ifdef LTTNG_USE_NMI_SAFE_CLOCK */
 138 static inline int get_trace_clock(void)
 139 {
 140         printk_once(KERN_WARNING "LTTng: Using mainline kernel monotonic clock. NMIs will not be traced.\n");
 141         return 0;
 142 }
 143 #endif /* #else #ifdef LTTNG_USE_NMI_SAFE_CLOCK */
 144
 145 static inline void put_trace_clock(void)
 146 {
 147 }
 148
 149 static inline u64 trace_clock_read64(void)
 150 {
 151         struct lttng_trace_clock *ltc = READ_ONCE(lttng_trace_clock);
 152
 153         if (likely(!ltc)) {
 154                 return trace_clock_read64_monotonic();
 155         } else {
 156                 read_barrier_depends(); /* load ltc before content */
 157                 return ltc->read64();
 158         }
 159 }
 160
 161 static inline u64 trace_clock_freq(void)
 162 {
 163         struct lttng_trace_clock *ltc = READ_ONCE(lttng_trace_clock);
 164
 165         if (!ltc) {
 166                 return trace_clock_freq_monotonic();
 167         } else {
 168                 read_barrier_depends(); /* load ltc before content */
 169                 return ltc->freq();
 170         }
 171 }
 172
 173 static inline int trace_clock_uuid(char *uuid)
 174 {
 175         struct lttng_trace_clock *ltc = READ_ONCE(lttng_trace_clock);
 176
 177         read_barrier_depends(); /* load ltc before content */
 178         /* Use default UUID cb when NULL */
 179         if (!ltc || !ltc->uuid) {
 180                 return trace_clock_uuid_monotonic(uuid);
 181         } else {
 182                 return ltc->uuid(uuid);
 183         }
 184 }
 185
 186 static inline const char *trace_clock_name(void)
 187 {
 188         struct lttng_trace_clock *ltc = READ_ONCE(lttng_trace_clock);
 189
 190         if (!ltc) {
 191                 return trace_clock_name_monotonic();
 192         } else {
 193                 read_barrier_depends(); /* load ltc before content */
 194                 return ltc->name();
 195         }
 196 }
 197
 198 static inline const char *trace_clock_description(void)
 199 {
 200         struct lttng_trace_clock *ltc = READ_ONCE(lttng_trace_clock);
 201
 202         if (!ltc) {
 203                 return trace_clock_description_monotonic();
 204         } else {
 205                 read_barrier_depends(); /* load ltc before content */
 206                 return ltc->description();
 207         }
 208 }
 209
 210 #endif /* CONFIG_HAVE_TRACE_CLOCK */
 211
 212 #endif /* _LTTNG_TRACE_CLOCK_H */