linux-kernel - Re: [BUG?] false positive in soft lockup detector while unlzma initramfs on slow cpu

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Date:	Tue, 29 Jan 2013 10:33:48 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Mike Lykov <combr@...dex.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org,
	kirill@...temov.name
Subject: Re: [BUG?] false positive in soft lockup detector while unlzma
 initramfs on slow cpu

Hi Mike,

On Tue, Jan 29, 2013 at 05:42:43PM +0400, Mike Lykov wrote:
> 
> So my questions:
> 
> 1. Are there a BUG in soft lockup detection mechanizm? Changing
> watchdog_thresh to 30 have a side effect in production - D-state
> userspace processes will be detected slowly. Are there a need to
> detecting soft lockups at boot time? Maybe it need after initramfs
> boot only when userspace processes begin to work?

The softlockup mechanism works scheduling a high priority task that kicks
the softlockups.  If the unzip thread is taking too long, it could
accidentally trip the detection.

Seeing that you are running on a 600 MHz machine, it could be possible.
Though I am not entirely sure how the scheduling works for decompressing
the initramfs.  I wouldn't think it is that high of a priority.

> 
> 2. How to change watchdog_thresh parameter at boot without patching
> sources? If it necessary (with it side effects) maybe implement it
> as commandline parameter or config compile time parameter?

I attached a patch below that allows you to set it a boot time.  Let me
know if this works for you, then I can clean it up and post it properly.

Cheers,
Don


diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 75a2ab3..e448d63 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -79,6 +79,14 @@ static int __init softlockup_panic_setup(char *str)
 }
 __setup("softlockup_panic=", softlockup_panic_setup);
 
+static int __init watchdog_thresh_setup(char *str)
+{
+	watchdog_thresh = simple_strtoul(str, NULL, 0);
+
+	return 1;
+}
+__setup("watchdog_thresh=", watchdog_thresh_setup);
+
 static int __init nowatchdog_setup(char *str)
 {
 	watchdog_enabled = 0;
--
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