RE: [perf probe] How to ask what variables can be collected at function return?

Date: Wed Sep 30 2015 - 04:05:28 EST

From: Arnaldo Carvalho de Melo [mailto:acme@xxxxxxxxxx]
>How to figure out which variables can I collect at function return?

At first, it seems the below pattern failed to get the probe point itself,
because you're specifying the return point of an inlined function, which
is nowhere.

If you give a function which has actual instance, perf-probe may show
the variables which will be accessable in the entry of the function as below.

$ sudo ./perf probe -V "vfs_read%return"
Available variables at vfs_read%return
char* buf
loff_t* pos
size_t count
struct file* file

However, this may not work on the arguments passed by registers,
since those may be already broken by the operations in the function.

So, to get correct accessible variables at return, you must give an actual
line number in the function currently.

$ sudo ./perf probe -L vfs_read
0 ssize_t vfs_read(struct file *file, char __user *buf, size_t count, lof
1 {
ssize_t ret;

4 if (!(file->f_mode & FMODE_READ))
5 return -EBADF;
6 if (!(file->f_mode & FMODE_CAN_READ))
7 return -EINVAL;
8 if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
9 return -EFAULT;

11 ret = rw_verify_area(READ, file, pos, count);
12 if (ret >= 0) {
count = ret;
14 ret = __vfs_read(file, buf, count, pos);
15 if (ret > 0) {
16 fsnotify_access(file);
17 add_rchar(current, ret);
19 inc_syscr(current);

return ret;
23 }

$ sudo ./perf probe -V vfs_read:23
Available variables at vfs_read:23
char* buf
loff_t* pos
size_t count
struct file* file

>I think we need a better error message :-)
>[root@zoo ~]# perf probe -V getname_flags%return Return probe must be on the head of a real function.

It seems that perf probe said " Return probe must be on the head of a real function."
So this means it can not find the given probe point because the %return is not
put on the entry of the function which has actual instance.


>Debuginfo analysis failed.
> Error: Failed to show vars.
>[root@zoo ~]#
>- Arnaldo
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