atrace

static void atrace_init_once()
{
    atrace_marker_fd = open("/sys/kernel/debug/tracing/trace_marker", O_WRONLY | O_CLOEXEC);

    if (atrace_marker_fd == -1) {
        ALOGE("Error opening trace file: %s (%d)", strerror(errno), errno);
        atrace_enabled_tags = 0;
    } else {
      atrace_enabled_tags = atrace_get_property();
    }
}
atrace_get_property-获取debug.atrace.tags.enableflags的值，默认为ATRACE_TAG_ALWAYS

// Read the sysprop and return the value tags should be set to
static uint64_t atrace_get_property()
{
    char value[PROPERTY_VALUE_MAX];
    char *endptr;
    uint64_t tags;

    property_get("debug.atrace.tags.enableflags", value, "0");
    errno = 0;
    tags = strtoull(value, &endptr, 0);
    if (value[0] == '\0' || *endptr != '\0') {
        ALOGE("Error parsing trace property: Not a number: %s", value);
        return 0;
    } else if (errno == ERANGE || tags == ULLONG_MAX) {
        ALOGE("Error parsing trace property: Number too large: %s", value);
        return 0;
    }

    // Only set the "app" tag if this process was selected for app-level debug
    // tracing.
    if (atrace_is_app_tracing_enabled()) {
        // 匹配的话，tags就有APP，APP的是第12位为1
        tags |= ATRACE_TAG_APP;
    } else {
        tags &= ~ATRACE_TAG_APP;
    }
// ATRACE_TAG_VALID_MASK表示前26位都为1，ATRACE_TAG_ALWAYS表示第1位为1
    return (tags | ATRACE_TAG_ALWAYS) & ATRACE_TAG_VALID_MASK;
}
#define ATRACE_BEGIN(name) atrace_begin(ATRACE_TAG, name)
static inline void atrace_begin(uint64_t tag, const char* name)
{// 为false，则打印
    if (CC_UNLIKELY(atrace_is_tag_enabled(tag))) {
        void atrace_begin_body(const char*);
        // 走这里
        atrace_begin_body(name);
    }
}
#define ATRACE_ENABLED() atrace_is_tag_enabled(ATRACE_TAG)
static inline uint64_t atrace_is_tag_enabled(uint64_t tag)
{// atrace_get_enabled_tags返回为ATRACE_TAG_ALWAYS，vold的话，tag就是pm，所以这里false
    return atrace_get_enabled_tags() & tag;
}
uint64_t atrace_get_enabled_tags()
{
    atrace_init();
    return atrace_enabled_tags;
}
void atrace_begin_body(const char* name)
{
    WRITE_MSG("B|%d|", "%s", name, "");
}
#define WRITE_MSG(format_begin, format_end, name, value) { \
    char buf[ATRACE_MESSAGE_LENGTH] __attribute__((uninitialized));     \
    int pid = getpid(); \
    int len = snprintf(buf, sizeof(buf), format_begin "%s" format_end, pid, \
        name, value); \
    if (len >= (int) sizeof(buf)) { \
        /* Given the sizeof(buf), and all of the current format buffers, \
         * it is impossible for name_len to be < 0 if len >= sizeof(buf). */ \
        int name_len = strlen(name) - (len - sizeof(buf)) - 1; \
        /* Truncate the name to make the message fit. */ \
        ALOGW("Truncated name in %s: %s\n", __FUNCTION__, name); \
        len = snprintf(buf, sizeof(buf), format_begin "%.*s" format_end, pid, \
            name_len, name, value); \
    } \
    write(atrace_marker_fd, buf, len); \
}