linux-kernel - Re: [PATCH rcu 11/12] torture: Flush printk() buffers before powering off

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Date:   Tue, 21 Jun 2022 11:13:35 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org
Subject: Re: [PATCH rcu 11/12] torture: Flush printk() buffers before
 powering off

On Tue, Jun 21, 2022 at 10:15:29AM +0206, John Ogness wrote:
> On 2022-06-20, "Paul E. McKenney" <paulmck@...nel.org> wrote:
> > But given Petr's and your recent changes that are now in mainline, is
> > this still really needed?  (Tests now pass without it, though tests can
> > of course be misleading.)
> 
> If all you are interested is if your rcutorture testing was successful,
> then I would keep this change. Then, no matter what the kernel does
> afterwards, you have your test results.
> 
> However, the kernel should correctly shutdown and it should flush the
> kernel buffers on shutdown or crash. (It is a kernel bug otherwise.) So
> if you also want to implicitly test this (non-rcutorture) functionality,
> then you should not use this patch.

Why not both?  ;-)

The patch below will cause rcutorture to implicitly test this
functionality, unless told otherwise, for example, by using the
--bootargs "torture.printk_shutdown_bug_workaround" kvm.sh
argument.

Thoughts?

							Thanx, Paul

------------------------------------------------------------------------

commit 204bf1e2a5a2fb68c15b4b64793ad0896db6f705
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Tue Jun 21 11:02:25 2022 -0700

    torture: Optionally flush printk() buffers before powering off
    
    The rcutorture test suite produces quite a bit of console output at
    the end of a test.  This means that the new-in-2022 printk() kthreads
    are likely to be in the process of flushing output at the time of the
    torture_shutdown() function's call to kernel_power_off().  Normally,
    rcutorture relies on printk() to flush any pending output upon shutdown,
    the better to detect bugs in this area, for example, the one introduced
    by 8e274732115f ("printk: extend console_lock for per-console locking").
    However, once such a bug is detected and reported, it is necessary to
    test the rest of the system, without noise from the already-reported bug.
    
    This commit therefore adds a torture.printk_shutdown_bug_workaround
    kernel parameter, which causes torture_shutdown() to invoke pr_flush(),
    and print an informative message on the console, immediately before
    invoking kernel_power_off().  When this kernel parameter is not specified,
    it is up to printk() to flush its own buffers.
    
    Suggested-by: John Ogness <john.ogness@...utronix.de>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d8de18d8f8875..652f49c4e5990 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6045,6 +6045,12 @@
 			are running concurrently, especially on systems
 			with rotating-rust storage.
 
+	torture.printk_shutdown_bug_workaround= [KNL]
+			Execute pr_flush(1000, true) just before invoking
+			kernel_power_off() to work around any bugs that
+			might prevent printk() from flushing its buffers
+			at shutdown time.
+
 	torture.verbose_sleep_frequency= [KNL]
 			Specifies how many verbose printk()s should be
 			emitted between each sleep.  The default of zero
diff --git a/kernel/torture.c b/kernel/torture.c
index 789aeb0e1159c..7cd2016b02076 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -48,6 +48,9 @@ module_param(disable_onoff_at_boot, bool, 0444);
 static bool ftrace_dump_at_shutdown;
 module_param(ftrace_dump_at_shutdown, bool, 0444);
 
+static bool printk_shutdown_bug_workaround;
+module_param(printk_shutdown_bug_workaround, bool, 0444);
+
 static int verbose_sleep_frequency;
 module_param(verbose_sleep_frequency, int, 0444);
 
@@ -651,6 +654,10 @@ static int torture_shutdown(void *arg)
 		VERBOSE_TOROUT_STRING("No torture_shutdown_hook(), skipping.");
 	if (ftrace_dump_at_shutdown)
 		rcu_ftrace_dump(DUMP_ALL);
+	if (printk_shutdown_bug_workaround) {
+		pr_info("%s: Flushing printk() buffers at power-down time.\n", __func__);
+		pr_flush(1000, true);
+	}
 	kernel_power_off();	/* Shut down the system. */
 	return 0;
 }