On Fri, Dec 13, 2019 at 11:25:00AM -0500, Steven Rostedt wrote: > On Fri, 13 Dec 2019 16:35:53 +0100 > Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > I don't think dedup algorithm can handle this and I'm not sure if there's > > some way in pahole to detect/prevent this. > > > > I only found that if I rename the ring_buffer objects to have distinct > > names, it will help: > > > > $ bpftool btf dump file /sys/kernel/btf/vmlinux | grep task_struct > > [150] STRUCT 'task_struct' size=11008 vlen=205 > > > > $ bpftool btf dump file /sys/kernel/btf/vmlinux | grep "STRUCT 'perf_event'" > > [1665] STRUCT 'perf_event' size=1160 vlen=70 > > > > also the BTF data get smaller ;-) before: > > > > $ ll /sys/kernel/btf/vmlinux > > -r--r--r--. 1 root root 2067432 Dec 13 22:56 /sys/kernel/btf/vmlinux > > > > after: > > $ ll /sys/kernel/btf/vmlinux > > -r--r--r--. 1 root root 1984345 Dec 13 23:02 /sys/kernel/btf/vmlinux > > > > > > Peter, Steven, > > if above is correct and there's no other better solution, would it be possible > > to straighten up the namespace and user some distinct names for perf and ftrace > > ring buffers? > > Now, the ring buffer that ftrace uses is not specific for ftrace or > even tracing for that matter. It is a stand alone ring buffer (oprofile > uses it), and can be used by anyone else. > > As the perf ring buffer is very coupled with perf (or perf events), and > unless something changed, I was never able to pull the perf ring > buffer out as a stand alone ring buffer. > > As the ring buffer in the tracing directory is more generic, and not to > mention around longer, if one is to change the name, I would suggest it > be the perf ring buffer. Thanks Jiri to bring it up. Technically the kernel is written in valid C, but it's imo the case where C standard isn't doing that great. To rephrase what Jiri said... If include/linux/perf_event.h instead of saying 'struct ring_buffer;' would have done #include <kernel/events/internal.h> the kernel/trace/ring_buffer.c would have had build error. Even now sizeof(*perf_event->rb) would return different values depending whether it's used in kernel/trace/ring_buffer.c or anywhere in kernel/events/. dwarf/btf and gcc cannot deal with it nicely. It's a valid C. 'gdb vmlinux' for sizeof(struct ring_buffer) will pick perf ring buffer. It had two choices. Both valid. I don't know why gdb picked this one. So yeah I think renaming 'ring_buffer' either in ftrace or in perf would be good. I think renaming ftrace one would be better, since gdb picked perf one for whatever reason.
