* Ingo Molnar <mi...@kernel.org> wrote:
> So if an overwrite-mode background tracing session is running, you don't even
> have
> to signal it to capture the ring-buffer: just open the ring-buffer fd in
> procfs,
> under /proc/XYZ/perf/ring-buffers/5.trace or so, and dump its current
> contents,
> assuming the task doing that has sufficient permissions - i.e.
> ptrace_may_access().
>
> We could even pretty-print some very basic version of the records from the
> kernel,
> via /proc/XYZ/perf/ring-buffers/5.txt, to support a tooling-less tracing
> modes.
> This way perf based tracing could be supported even on systems that have no
> writable filesystems.
With this 'cat' could be used to look at the current trace:
cat /proc/XYZ/perf/ring-buffers/5.txt
would result in 'perf script' alike output generated by the kernel:
$ cat /proc/XYZ/perf/ring-buffers/5.txt
perf 24816 63796.780079: 1 cycles:pp: ffffffff810603f8
native_write_msr_safe
perf 24816 63796.780083: 1 cycles:pp: ffffffff810603f8
native_write_msr_safe
perf 24816 63796.780086: 8 cycles:pp: ffffffff810603f8
native_write_msr_safe
perf 24816 63796.780089: 97 cycles:pp: ffffffff810603f8
native_write_msr_safe
perf 24816 63796.780092: 1237 cycles:pp: ffffffff8103450c
intel_pmu_handle_irq
perf 24816 63796.780094: 13879 cycles:pp: ffffffff81204f23
setup_new_exec
sh 24816 63796.780104: 170378 cycles:pp: ffffffff811bc437
change_protection_range
sh 24816 63796.780145: 698206 cycles:pp: ffffffff813d22d7
clear_page_c_e
sh 24816 63796.780304: 1145748 cycles:pp: 7f60aca20bdb
_dl_addr
sh 24817 63796.780400: 1 cycles:pp: ffffffff810603f8
native_write_msr_safe
sh 24817 63796.780403: 1 cycles:pp: ffffffff810603f8
native_write_msr_safe
sh 24817 63796.780406: 10 cycles:pp: ffffffff810603f8
native_write_msr_safe
sh 24817 63796.780409: 118 cycles:pp: ffffffff810603f8
native_write_msr_safe
... and you could use shell scripting to analyze it - just like with ftrace.
of course this would be simplified output - and you could still access or copy
the
raw trace data as well, and use all the rich tooling and visualization features
of
full perf.
Thanks,
Ingo
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