torture: Update initrd documentation

The rcu-test-image.txt documentation covers a very uncommon case where
a real userspace environment is required.  However, someone reading this
document might reasonably conclude that this is in fact a prerequisite.
In addition, the initrd.txt file mentions dracut, which is no longer used.
This commit therefore provides the needed updates.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 30 insertions, 5 deletions

diff --git a/tools/testing/selftests/rcutorture/doc/rcu-test-image.txt b/tools/testing/selftests/rcutorture/doc/rcu-test-image.txt
index cc280ba157a3..b2fc247976b1 100644
--- a/tools/testing/selftests/rcutorture/doc/rcu-test-image.txt
+++ b/tools/testing/selftests/rcutorture/doc/rcu-test-image.txt
@@ -1,8 +1,33 @@
-This document describes one way to create the rcu-test-image file
-that contains the filesystem used by the guest-OS kernel.  There are
-probably much better ways of doing this, and this filesystem could no
-doubt be smaller.  It is probably also possible to simply download
-an appropriate image from any number of places.
+Normally, a minimal initrd is created automatically by the rcutorture
+scripting.  But minimal really does mean "minimal", namely just a single
+root directory with a single statically linked executable named "init":
+
+$ size tools/testing/selftests/rcutorture/initrd/init
+   text    data     bss     dec     hex filename
+    328       0       8     336     150 tools/testing/selftests/rcutorture/initrd/init
+
+Suppose you need to run some scripts, perhaps to monitor or control
+some aspect of the rcutorture testing.  This will require a more fully
+filled-out userspace, perhaps containing libraries, executables for
+the shell and other utilities, and soforth.  In that case, place your
+desired filesystem here:
+
+	tools/testing/selftests/rcutorture/initrd
+
+For example, your tools/testing/selftests/rcutorture/initrd/init might
+be a script that does any needed mount operations and starts whatever
+scripts need starting to properly monitor or control your testing.
+The next rcutorture build will then incorporate this filesystem into
+the kernel image that is passed to qemu.
+
+Or maybe you need a real root filesystem for some reason, in which case
+please read on!
+
+The remainder of this document describes one way to create the
+rcu-test-image file that contains the filesystem used by the guest-OS
+kernel.  There are probably much better ways of doing this, and this
+filesystem could no doubt be smaller.  It is probably also possible to
+simply download an appropriate image from any number of places.
 
 That said, here are the commands: