perf trace: substruct BTF based pretty printing

[Arnaldo Carvalho de Melo]

Hi Howard,

	Not really a requirement on you to do work, just a some notes to
add to our discussion/experiment on using BTF to pretty print syscall
(and tracepoints/whatever) arguments:

root@number:~# perf trace -e setitimer -p 5444 |& head -5
     0.000 ( 0.017 ms): Xwayland/5444 setitimer(value: (struct __kernel_old_itimerval){})                   = 0
     0.050 ( 0.004 ms): Xwayland/5444 setitimer(value: (struct __kernel_old_itimerval){})                   = 0
     0.142 ( 0.005 ms): Xwayland/5444 setitimer(value: (struct __kernel_old_itimerval){})                   = 0
     0.174 ( 0.004 ms): Xwayland/5444 setitimer(value: (struct __kernel_old_itimerval){})                   = 0
     0.293 ( 0.004 ms): Xwayland/5444 setitimer(value: (struct __kernel_old_itimerval){})                   = 0
root@number:~# strace -e setitimer -p 5444 |& head -5
strace: Process 5444 attached
setitimer(ITIMER_REAL, {it_interval={tv_sec=0, tv_usec=5000}, it_value={tv_sec=0, tv_usec=5000}}, NULL) = 0
setitimer(ITIMER_REAL, {it_interval={tv_sec=0, tv_usec=0}, it_value={tv_sec=0, tv_usec=0}}, NULL) = 0
setitimer(ITIMER_REAL, {it_interval={tv_sec=0, tv_usec=5000}, it_value={tv_sec=0, tv_usec=5000}}, NULL) = 0
setitimer(ITIMER_REAL, {it_interval={tv_sec=0, tv_usec=0}, it_value={tv_sec=0, tv_usec=0}}, NULL) = 0
root@number:~# 
root@number:~# 
root@number:~# grep -w value /sys/kernel/tracing/events/syscalls/sys_enter_rseq/format 
root@number:~# grep -w value /sys/kernel/tracing/events/syscalls/sys_enter_setitimer/format 
	field:struct __kernel_old_itimerval * value;	offset:24;	size:8;	signed:0;
print fmt: "which: 0x%08lx, value: 0x%08lx, ovalue: 0x%08lx", ((unsigned long)(REC->which)), ((unsigned long)(REC->value)), ((unsigned long)(REC->ovalue))
root@number:~# pahole __kernel_old_itimerval
struct __kernel_old_itimerval {
	struct __kernel_old_timeval it_interval;         /*     0    16 */
	struct __kernel_old_timeval it_value;            /*    16    16 */

	/* size: 32, cachelines: 1, members: 2 */
	/* last cacheline: 32 bytes */
};

root@number:~# pahole -E __kernel_old_itimerval
struct __kernel_old_itimerval {
	struct __kernel_old_timeval {
		/* typedef __kernel_long_t */ long int           tv_sec;                 /*     0     8 */
		/* typedef __kernel_long_t */ long int           tv_usec;                /*     8     8 */
	} it_interval; /*     0    16 */
	struct __kernel_old_timeval {
		/* typedef __kernel_long_t */ long int           tv_sec;                 /*    16     8 */
		/* typedef __kernel_long_t */ long int           tv_usec;                /*    24     8 */
	} it_value; /*    16    16 */

	/* size: 32, cachelines: 1, members: 2 */
	/* last cacheline: 32 bytes */
};

root@number:~#