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Date: Fri, 01 Nov 2013 07:39:21 -0400
From: Prarit Bhargava <prarit@...hat.com>
To: Mel Gorman <mgorman@...e.de>
CC: Rik van Riel <riel@...hat.com>, peterz@...radead.org,
mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip] fix race between stop_two_cpus and stop_cpus
On 11/01/2013 07:08 AM, Mel Gorman wrote:
> On Thu, Oct 31, 2013 at 04:31:44PM -0400, Rik van Riel wrote:
>> There is a race between stop_two_cpus, and the global stop_cpus.
>>
>
> What was the trigger for this? I want to see what was missing from my own
> testing. I'm going to go out on a limb and guess that CPU hotplug was also
> running in the background to specifically stress this sort of rare condition.
> Something like running a standard test with the monitors/watch-cpuoffline.sh
> from mmtests running in parallel.
>
I have a test that loads and unloads each module in /lib/modules/3.*/...
Each run typically takes a few minutes. After running 4-5 times, the system
issues a soft lockup warning with a CPU in multi_cpu_stop(). Unfortunately,
kdump isn't working on this particular system (due to another bug) so I modified
the code with (sorry for the cut-and-paste):
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 05039e3..4a8c9f9 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -323,8 +323,10 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtime
else
dump_stack();
- if (softlockup_panic)
+ if (softlockup_panic) {
+ show_state();
panic("softlockup: hung tasks");
+ }
__this_cpu_write(soft_watchdog_warn, true);
} else
__this_cpu_write(soft_watchdog_warn, false);
and then 'echo 1 > /proc/sys/kernel/softlockup_panic' to get a full trace of all
tasks.
When I did this and ran the kernel module load unload test ...
[prarit@...rit tmp]$ cat /tmp/intel.log | grep RIP
[ 678.081168] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.156180] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.230190] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.244186] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.259194] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.274192] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.288195] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.303197] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.318200] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.333203] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.349206] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.364208] RIP: 0010:[<ffffffff810d328b>] [<ffffffff810d328b>]
multi_cpu_stop+0x7b/0xf0
[ 678.379211] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.394212] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.409215] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.424217] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.438219] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.452221] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.466228] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.481228] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.496230] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.511234] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.526236] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.541238] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.556244] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.571243] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.586247] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
[ 678.601248] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.616251] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 678.632254] RIP: 0010:[<ffffffff810d328b>] [<ffffffff810d328b>]
multi_cpu_stop+0x7b/0xf0
[ 678.647257] RIP: 0010:[<ffffffff810d3292>] [<ffffffff810d3292>]
multi_cpu_stop+0x82/0xf0
[ 687.570464] RIP: 0010:[<ffffffff810d3296>] [<ffffffff810d3296>]
multi_cpu_stop+0x86/0xf0
and,
[prarit@...rit tmp]$ cat /tmp/intel.log | grep RIP | wc -l
32
which shows all 32 cpus are "correctly" in the cpu stop threads. After some
investigation, Rik came up with his patch.
Hope this explains things,
P.
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