tracing: Ensure trace buffer is at least 4096 bytes large

Booting the kernel with 'trace_buf_size=1' give a warning at
boot during the ftrace selftests:

[    0.892809] Running postponed tracer tests:
[    0.892893] Testing tracer function:
[    0.901899] Callback from call_rcu_tasks_trace() invoked.
[    0.983829] Callback from call_rcu_tasks_rude() invoked.
[    1.072003] .. bad ring buffer .. corrupted trace buffer ..
[    1.091944] Callback from call_rcu_tasks() invoked.
[    1.097695] PASSED
[    1.097701] Testing dynamic ftrace: .. filter failed count=0 ..FAILED!
[    1.353474] ------------[ cut here ]------------
[    1.353478] WARNING: CPU: 0 PID: 1 at kernel/trace/trace.c:1951 run_tracer_selftest+0x13c/0x1b0

Therefore enforce a minimum of 4096 bytes to make the selftest pass.

Link: https://lkml.kernel.org/r/20220214134456.1751749-1-svens@linux.ibm.com
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 7c2578e..3050892 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1474,10 +1474,12 @@ static int __init set_buf_size(char *str)
        if (!str)
                return 0;
        buf_size = memparse(str, &str);
-       /* nr_entries can not be zero */
-       if (buf_size == 0)
-               return 0;
-       trace_buf_size = buf_size;
+       /*
+        * nr_entries can not be zero and the startup
+        * tests require some buffer space. Therefore
+        * ensure we have at least 4096 bytes of buffer.
+        */
+       trace_buf_size = max(4096UL, buf_size);
        return 1;
 }
 __setup("trace_buf_size=", set_buf_size);