zeros in frame pointer callchains

Hi,

Namhyung, while working on supporting callchains in 'perf trace' I
noticed something I think we discussed at some point, zeros in frame pointer callchains...

Oh well figured it out, its probably userspace not having frame
pointers, grrr anyway, keeping the rest of the message, for reference, but now I think we need to have some expressiveness to say that for that particular event we want to use perf_event_attr.exclude_callchain_user...

First add a special probe point that 'perf trace' uses, if present:

  # perf probe 'vfs_getname=getname_flags:72 pathname=filename:string'

Then lets ask to see just when this thing hits, no syscall
printing whatsoever ('trace' == 'perf trace'), sometimes just a few zeros at the top, sometimes a long series, need to investigate how to chop those off in the kernel, not to use the ring buffer needlessly, will start chopping these in userspace (in perf trace):

  # trace --no-sys --ev probe:vfs_getname/call-graph=fp/

34033.533 probe:vfs_getname:(ffffffff8124c06c) pathname="/usr/share/pixmaps")

                                       getname_flags+0xfe20009d ([kernel.kallsyms])
                                       user_path_at_empty+0xfe200023 ([kernel.kallsyms])
                                       vfs_fstatat+0xfe200066 ([kernel.kallsyms])
                                       SYSC_newstat+0xfe20002e ([kernel.kallsyms])
                                       sys_newstat+0xfe20000e ([kernel.kallsyms])
                                       entry_SYSCALL_64_fastpath+0xfe20001a ([kernel.kallsyms])
                                       __xstat64+0xffff011e90b98015 (/usr/lib64/libc-2.22.so)
                                       [0] ([unknown])
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                                       [0] ([unknown])
 36001.681 probe:vfs_getname:(ffffffff8124c06c) pathname="/var/tmp")
                                       getname_flags+0xfe20009d ([kernel.kallsyms])
                                       user_path_at_empty+0xfe200023 ([kernel.kallsyms])
                                       vfs_fstatat+0xfe200066 ([kernel.kallsyms])
                                       SYSC_newstat+0xfe20002e ([kernel.kallsyms])
                                       sys_newstat+0xfe20000e ([kernel.kallsyms])
                                       entry_SYSCALL_64_fastpath+0xfe20001a ([kernel.kallsyms])
                                       __xstat64+0xffff00b816a28015 (/usr/lib64/libc-2.22.so)
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 36001.696 probe:vfs_getname:(ffffffff8124c06c) pathname="/var/tmp")
                                       getname_flags+0xfe20009d ([kernel.kallsyms])
                                       user_path_at_empty+0xfe200023 ([kernel.kallsyms])
                                       sys_access+0xfe2000b4 ([kernel.kallsyms])
                                       entry_SYSCALL_64_fastpath+0xfe20001a ([kernel.kallsyms])
                                       __GI___access+0xffff00b816a28007 (/usr/lib64/libc-2.22.so)
                                       [0] ([unknown])
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  8254.597 probe:vfs_getname:(ffffffff8124c06c) pathname="/etc/localtime")
                                       getname_flags+0xfe20009d ([kernel.kallsyms])
                                       user_path_at_empty+0xfe200023 ([kernel.kallsyms])
                                       vfs_fstatat+0xfe200066 ([kernel.kallsyms])
                                       SYSC_newstat+0xfe20002e ([kernel.kallsyms])
                                       sys_newstat+0xfe20000e ([kernel.kallsyms])
                                       entry_SYSCALL_64_fastpath+0xfe20001a ([kernel.kallsyms])
                                       __xstat64+0xffff0104f5696015 (/usr/lib64/libc-2.22.so)
                                       [0] ([unknown])
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  9001.567 probe:vfs_getname:(ffffffff8124c06c) pathname="/etc/firewalld/ipsets")
                                       getname_flags+0xfe20009d ([kernel.kallsyms])
                                       user_path_at_empty+0xfe200023 ([kernel.kallsyms])
                                       sys_inotify_add_watch+0xfe200098 ([kernel.kallsyms])
                                       entry_SYSCALL_64_fastpath+0xfe20001a ([kernel.kallsyms])
                                       inotify_add_watch+0xffff008bb9432007 (/usr/lib64/libc-2.22.so)
  9450.181 probe:vfs_getname:(ffffffff8124c06c) pathname="/root")
                                       getname_flags+0xfe20009d ([kernel.kallsyms])
                                       user_path_at_empty+0xfe200023 ([kernel.kallsyms])
                                       sys_inotify_add_watch+0xfe200098 ([kernel.kallsyms])
                                       entry_SYSCALL_64_fastpath+0xfe20001a ([kernel.kallsyms])
                                       inotify_add_watch+0xffff01b7aa4f2007 (/usr/lib64/libc-2.22.so)

Hi Arnaldo,

My solution was to look at /proc/sys/vm/mmap_min_addr [1]. For kernel it'd be simply discarding callchains after seeing an address below the mmap_min_addr. For userspace, it requires to carry the mmap_min_addr via feature bit or something. Or Maybe we can just discard addresses under 4096 for simplicity?

[1] https://lkml.org/lkml/2015/11/26/40

Thanks,
Namhyung