From 20e7168326f5ccab0fc5c322af31ae6200012137 Mon Sep 17 00:00:00 2001 From: "Masami Hiramatsu (Google)" Date: Sat, 22 Nov 2025 09:31:40 +0900 Subject: tracing: Add boot-time backup of persistent ring buffer Currently, the persistent ring buffer instance needs to be read before using it. This means we have to wait for boot up user space and dump the persistent ring buffer. However, in that case we can not start tracing on it from the kernel cmdline. To solve this limitation, this adds an option which allows to create a trace instance as a backup of the persistent ring buffer at boot. If user specifies trace_instance== then the instance is made as a copy of the instance. For example, the below kernel cmdline records all syscalls, scheduler and interrupt events on the persistent ring buffer `boot_map` but before starting the tracing, it makes a `backup` instance from the `boot_map`. Thus, the `backup` instance has the previous boot events. 'reserve_mem=12M:4M:trace trace_instance=boot_map@trace,syscalls:*,sched:*,irq:* trace_instance=backup=boot_map' As you can see, this just make a copy of entire reserved area and make a backup instance on it. So you can release (or shrink) the backup instance after use it to save the memory usage. /sys/kernel/tracing/instances # free total used free shared buff/cache available Mem: 1999284 55704 1930520 10132 13060 1914628 Swap: 0 0 0 /sys/kernel/tracing/instances # rmdir backup/ /sys/kernel/tracing/instances # free total used free shared buff/cache available Mem: 1999284 40640 1945584 10132 13060 1929692 Swap: 0 0 0 Note: since there is no reason to make a copy of empty buffer, this backup only accepts a persistent ring buffer as the original instance. Also, since this backup is based on vmalloc(), it does not support user-space mmap(). Cc: Mathieu Desnoyers Link: https://patch.msgid.link/176377150002.219692.9425536150438129267.stgit@devnote2 Signed-off-by: Masami Hiramatsu (Google) Signed-off-by: Steven Rostedt (Google) --- kernel/trace/trace.h | 1 + 1 file changed, 1 insertion(+) (limited to 'kernel/trace/trace.h') diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h index a3aa225ed50a..666f9a2c189d 100644 --- a/kernel/trace/trace.h +++ b/kernel/trace/trace.h @@ -454,6 +454,7 @@ enum { TRACE_ARRAY_FL_LAST_BOOT = BIT(2), TRACE_ARRAY_FL_MOD_INIT = BIT(3), TRACE_ARRAY_FL_MEMMAP = BIT(4), + TRACE_ARRAY_FL_VMALLOC = BIT(5), }; #ifdef CONFIG_MODULES -- cgit v1.2.3