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Diffstat (limited to 'Documentation/trace/debugging.rst')
| -rw-r--r-- | Documentation/trace/debugging.rst | 23 |
1 files changed, 22 insertions, 1 deletions
diff --git a/Documentation/trace/debugging.rst b/Documentation/trace/debugging.rst index 54fb16239d70..bca1710d92bf 100644 --- a/Documentation/trace/debugging.rst +++ b/Documentation/trace/debugging.rst @@ -59,7 +59,7 @@ There is various methods of acquiring the state of the system when a kernel crash occurs. This could be from the oops message in printk, or one could use kexec/kdump. But these just show what happened at the time of the crash. It can be very useful in knowing what happened up to the point of the crash. -The tracing ring buffer, by default, is a circular buffer than will +The tracing ring buffer, by default, is a circular buffer that will overwrite older events with newer ones. When a crash happens, the content of the ring buffer will be all the events that lead up to the crash. @@ -136,6 +136,8 @@ kernel, so only the same kernel is guaranteed to work if the mapping is preserved. Switching to a different kernel version may find a different layout and mark the buffer as invalid. +NB: Both the mapped address and size must be page aligned for the architecture. + Using trace_printk() in the boot instance ----------------------------------------- By default, the content of trace_printk() goes into the top level tracing @@ -157,3 +159,22 @@ If setting it from the kernel command line, it is recommended to also disable tracing with the "traceoff" flag, and enable tracing after boot up. Otherwise the trace from the most recent boot will be mixed with the trace from the previous boot, and may make it confusing to read. + +Using a backup instance for keeping previous boot data +------------------------------------------------------ + +It is also possible to record trace data at system boot time by specifying +events with the persistent ring buffer, but in this case the data before the +reboot will be lost before it can be read. This problem can be solved by a +backup instance. From the kernel command line:: + + reserve_mem=12M:4096:trace trace_instance=boot_map@trace,sched,irq trace_instance=backup=boot_map + +On boot up, the previous data in the "boot_map" is copied to the "backup" +instance, and the "sched:*" and "irq:*" events for the current boot are traced +in the "boot_map". Thus the user can read the previous boot data from the "backup" +instance without stopping the trace. + +Note that this "backup" instance is readonly, and will be removed automatically +if you clear the trace data or read out all trace data from the "trace_pipe" +or the "trace_pipe_raw" files. |