include/linux/trace_events.h

The trace_entry struct represents the most basic unit of tracing.

struct trace_entry {
   unsigned short type;
   unsigned char  flags;
   unsigned char  preempt_count;
   int            pid;
};

struct trace_event_call {
        struct list_head          list;
        struct trace_event_class *class;
        union {
                char                    *name;
             /* Set TRACE_EVENT_FL_TRACEPOINT flag when using "tp" */
                struct tracepoint       *tp;
        };

        struct trace_event      event;
        char                    *print_fmt;
        struct event_filter     *filter;

        /*
           Static events can disappear with modules,
           where as dynamic ones need their own ref count.
         */
        union {
                void                            *module;
                atomic_t                        refcnt;
        };
        void                    *data;

        int                     flags; /* static flags of different events, see TRACE_EVENT_FL_* 'above' */

#ifdef CONFIG_PERF_EVENTS
        int                              perf_refcount;
        struct hlist_head __percpu      *perf_events;
        struct bpf_prog_array __rcu     *prog_array;

        int     (*perf_perm)(struct trace_event_call *, struct perf_event *);
#endif
};

A pointer to the struct trace_event_call is used as members enter_event and exit_event in struct syscall_metadata.