linux-kernel - Re: [PATCH 6/9] watchdog: implement error handling for failure to set up hardware perf events

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Date:	Mon, 2 Mar 2015 14:06:33 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 6/9] watchdog: implement error handling for failure to
 set up hardware perf events

Hi Andrew,

Here is another patch that can be folded into:

watchdog-implement-error-handling-for-failure-to-set-up-hardware-perf-events.patch

Let me know if you want me to fold this into the original and repost or not.

Cheers,
Don


----------------8<-------------------
From: Don Zickus <dzickus@...hat.com>
Date: Mon, 2 Mar 2015 13:56:23 -0500
Subject: [PATCH 2/2] watchdog: Update comments to explain some code

This patch was written to update some comments and concerns from Andrew and is
expected to be folded into patch

watchdog-implement-error-handling-for-failure-to-set-up-hardware-perf-events.patch

Resolves:
- comments around barriers
- comments around blind shutdown of hardware lockup detector
- printk letting someone know hardlockup detector is being shut down

Suggested-by: Ulrich Obergfell <uobergfe@...hat.com>
Signed-off-by: Don Zickus <dzickus@...hat.com>
---
 kernel/watchdog.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 55c2a4f..f2be11a 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -495,6 +495,12 @@ static void watchdog(unsigned int cpu)
 	 * failure path. Check for failures that can occur asynchronously -
 	 * for example, when CPUs are on-lined - and shut down the hardware
 	 * perf event on each CPU accordingly.
+	 *
+	 * The only non-obvious place this bit can be cleared is through
+	 * watchdog_nmi_enable(), so a pr_info() is placed there.  Placing a
+	 * pr_info here would be too noisy as it would result in a message
+	 * every few seconds if the hardlockup was disabled but the softlockup
+	 * enabled.
 	 */
 	if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
 		watchdog_nmi_disable(cpu);
@@ -546,6 +552,9 @@ static int watchdog_nmi_enable(unsigned int cpu)
 	 * Disable the hard lockup detector if _any_ CPU fails to set up
 	 * set up the hardware perf event. The watchdog() function checks
 	 * the NMI_WATCHDOG_ENABLED bit periodically.
+	 *
+	 * The barriers are for syncing up watchdog_enabled across all the
+	 * cpus, as clear_bit() does not use barriers.
 	 */
 	smp_mb__before_atomic();
 	clear_bit(NMI_WATCHDOG_ENABLED_BIT, &watchdog_enabled);
@@ -564,6 +573,9 @@ static int watchdog_nmi_enable(unsigned int cpu)
 	else
 		pr_err("disabled (cpu%i): unable to create perf event: %ld\n",
 			cpu, PTR_ERR(event));
+
+	pr_info("Shutting down hard lockup detector on all cpus\n");
+
 	return PTR_ERR(event);
 
 	/* success path */
-- 
1.7.1

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