missing mmap events in perf profiles without dwarf callgraph

[Milian Wolff <mail@milianw.de>]

[-- Attachment #1: Type: text/plain, Size: 2789 bytes --]

Hey there,

in hotspot I noticed a symbolization bug for perf.data files that are recorded 
without `--call-graph dwarf`, i.e. something like this:

```
$ cat test.c
int main()
{
    int sum = 0;
    for (int i = 0; i < 100000; ++i) sum += i;
    return sum > 0;
}
$ gcc -O0 -g test.c
$ perf record ./a.out
$ perf script --show-mmap-events | grep /a.out
           a.out 149719 18106.394277: PERF_RECORD_MMAP2 149719/149719: 
[0x55e0b3b5e000(0x1000) @ 0 00:31 22086 3699042272]: r-xp /tmp/a.out
           a.out 149719 18106.394748:     275254 cycles:u:      55e0b3b5e72e 
main+0x25 (/tmp/a.out)
           a.out 149719 18106.395011:     486078 cycles:u:      55e0b3b5e72e 
main+0x25 (/tmp/a.out)
```

How does perf resolve the address 0x55e0b3b5e72e to main+0x25 here? If we look 
at this purely from the singular mmap event, this shouldn't be the case:

1) The mmap event starts at 0x55e0b3b5e000, has size 0x1000, so its end is at 
0x55e0b3b5f000.

2) Address 55e0b3b5e72e is thus contained in this map, and at offset 0x72e.

3) But main is not at this offset, it starts at 0x1709:

```
$ nm ./a.out | grep main
                 U __libc_start_main
0000000000001709 T main
```

How does perf report/script still get the right answer here and knows that 
there's an offset of 0x1000?

Note that in reality, multiple mmap events occur. And the first one maps the 
file at an address with the 0x1000 offset. This can be seen by:

```
$ perf record --call-graph dwarf ./a.out
$ perf script --show-mmap-events | grep /a.out
a.out 149953 18488.265340: PERF_RECORD_MMAP2 149953/149953: 
[0x55b2f29d2000(0x4000) @ 0 00:31 22086 3699042272]: r--p /tmp/a.out
a.out 149953 18488.265346: PERF_RECORD_MMAP2 149953/149953: 
[0x55b2f29d3000(0x1000) @ 0 00:31 22086 3699042272]: r-xp /tmp/a.out
a.out 149953 18488.265348: PERF_RECORD_MMAP2 149953/149953: 
[0x55b2f29d4000(0x1000) @ 0 00:31 22086 3699042272]: rw-p /tmp/a.out
a.out 149953 18488.265350: PERF_RECORD_MMAP2 149953/149953: 
[0x55b2f29d5000(0x1000) @ 0 00:31 22086 3699042272]: rw-p /tmp/a.out
a.out 149953 18488.265706: PERF_RECORD_MMAP2 149953/149953: 
[0x55b2f29d4000(0x1000) @ 0 00:31 22086 3699042272]: r--p /tmp/a.out
            55b2f29d372e main+0x25 (/tmp/a.out)
            55b2f29d372e main+0x25 (/tmp/a.out)
```

In this case, we see that the first mmap event happens at 0x55b2f29d2000, so 
if we take that as the base address, we arrive at the offset 0x172e for 
address 0x55b2f29d372e. This then indeed matches main+0x25, i.e.: 0x1709 + 
0x25 = 0x172e.

But... The question again is: How does perf stat/report know this fact with 
just the singular mmap event in the plain `perf record a.out` case above?
Why is there just one mmap event recorded in such a profile?

Thank you
-- 
Milian Wolff
mail@milianw.de
http://milianw.de

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 833 bytes --]