Re: [PATCH v5 4/4] tracing/Documentation: Add a section about backup instance
From: Steven Rostedt <rostedt@goodmis.org>
Date: 2026-01-29 20:18:50
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On Wed, 28 Jan 2026 09:10:11 +0900 "Masami Hiramatsu (Google)" [off-list ref] wrote:
quoted hunk ↗ jump to hunk
From: Masami Hiramatsu (Google) <mhiramat@kernel.org> Add a section about backup instance to the debugging.rst. Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> --- Documentation/trace/debugging.rst | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+)diff --git a/Documentation/trace/debugging.rst b/Documentation/trace/debugging.rst index 4d88c346fc38..73e2154bcf98 100644 --- a/Documentation/trace/debugging.rst +++ b/Documentation/trace/debugging.rst@@ -159,3 +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 + +When the boot up, the previous data in the`boot_map` is copied to "backup"
s/When the boot up/On boot up/ copied to the "backup"
+instance, and the "sched:*" and "irq:*" events for current boot are traced on
for the current boot are traced in
+"boot_map". Thus user can read the previous boot data from the "backup" instance
Thus the user
+without stopping trace.
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 "trace_pipe" or +"trace_pipe_raw" files. \ No newline at end of file
Definite articles are a pain ;-) -- Steve