Re: [PATCH] perf record: Add snapshot mode support for perf's regular events

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@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