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

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Message-ID: <20130130154033.GE98867@redhat.com>
Date:	Wed, 30 Jan 2013 10:40:33 -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

On Wed, Jan 30, 2013 at 01:39:23PM +0400, Mike Lykov wrote:
> 29.01.2013 19:33, Don Zickus пишет:
> 
> >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.
> 
> Inyerestingly, that a decompress of lzma -4 takes longer time than
> decompress lzma -9, and it stated in man lzma(1):
> "  On  the  same hardware, the decompression speed is approximately
> a constant number of bytes of compressed data per second.  In other
> words, the better the compression, the faster the  decompression
> will  usually  be. "
> 
> I tested it on target computer by hand:
> 
> lzma -4 compressed: time unlzma initram-alt-p6rel3-4.cpio.lzma
> 20.94user 1.47system 0:22:45elapsed 99%CPU (...19424maxresidents)k
> 
> lzma -9 compressed: time unlzma initram-alt-p6rel3-9.cpio.lzma
> 19.49user 1.92system 0:21:44elapsed 99%CPU (...241488maxresidents)k
> 
> So, it cannot "take too long" because not-working faster than working.
> Apparently time not matter, but algorithm complexity?
> 
> >>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.
> 
> It not works for me. I apply this patch, build, use ("int
> __read_mostly watchdog_thresh = 10;"  as in original)
> command line:
> 
> [    0.000000] Kernel command line:
> initrd=initram-alt-p6rel3-9.cpio.lzma console=uart,io,0x240,115200n8
> kernel.watchdog_thresh=30

I have never seen usage like 'kernel.watchdog_thresh=30'.  Could you try
'watchdog_thresh=30' instead?

I also attached another patch as suggested by Andrew to add a
touch_softlockup_watchdog in the unlzma routine.  Probably makes things
run a little slower.  Compiled tested only.

Cheers,
Don

diff --git a/lib/decompress_unlzma.c b/lib/decompress_unlzma.c
index 32adb73..313f4fa 100644
--- a/lib/decompress_unlzma.c
+++ b/lib/decompress_unlzma.c
@@ -36,6 +36,7 @@
 #endif /* STATIC */
 
 #include <linux/decompress/mm.h>
+#include <linux/nmi.h>
 
 #define	MIN(a, b) (((a) < (b)) ? (a) : (b))
 
@@ -648,6 +649,7 @@ STATIC inline int INIT unlzma(unsigned char *buf, int in_len,
 		}
 		if (rc.buffer_size <= 0)
 			goto exit_3;
+		touch_softlockup_watchdog();
 	}
 
 	if (posp)
--
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