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Date: Wed, 4 Jun 2008 09:00:36 -0500
From: Dimitri Sivanich <sivanich@sgi.com>
To: linux-kernel@vger.kernel.org
Cc: Max Krasnyansky <maxk@qualcomm.com>, Paul Jackson <pj@sgi.com>,
       Con Kolivas <kernel@kolivas.org>, "Derek L. Fults" <dfults@sgi.com>,
       devik <devik@cdi.cz>, Dinakar Guniguntala <dino@in.ibm.com>,
       Emmanuel Pacaud <emmanuel.pacaud@univ-poitiers.fr>,
       Frederik Deweerdt <deweerdt@free.fr>, Ingo Molnar <mingo@elte.hu>,
       Matthew Dobson <colpatch@us.ibm.com>,
       Nick Piggin <nickpiggin@yahoo.com.au>, rostedt@goodmis.org,
       Oleg Nesterov <oleg@tv-sign.ru>,
       "Paul E. McKenney" <paulmck@us.ibm.com>,
       Paul Menage <menage@google.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       "Randy.Dunlap" <rddunlap@osdl.org>, suresh.b.siddha@intel.com,
       Thomas Gleixner <tglx@linutronix.de>
Subject: Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may have realtime uses)
Message-ID: <20080604140036.GC18993@sgi.com>
References: <20080601213019.14ea8ef8.pj@sgi.com> <20080602164203.GA2477@sgi.com> <48443E66.6060205@qualcomm.com> <20080602214151.GA7072@sgi.com> <48446D46.2010903@qualcomm.com> <20080603144010.GA25948@sgi.com>
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On Tue, Jun 03, 2008 at 09:40:10AM -0500, Dimitri Sivanich wrote:
> I tried the following scenario on an ia64 Altix running 2.6.26-rc4 with cpusets compiled in but cpuset fs unmounted.  Do your patches already address this?
> 
> $ taskset -cp 3 $$   (attach to cpu 3)
> pid 4591's current affinity list: 0-3
> pid 4591's new affinity list: 3
> $ echo 0 > /sys/devices/system/cpu/cpu2/online  (down cpu 2)
>   (above command hangs)
> 
> Backtrace of pid 4591 (bash)
> 
> Call Trace:
>  [<a00000010078e990>] schedule+0x1210/0x13c0
>                                 sp=e0000060b6dffc90 bsp=e0000060b6df11e0
>  [<a00000010078ef60>] schedule_timeout+0x40/0x180
>                                 sp=e0000060b6dffce0 bsp=e0000060b6df11b0
>  [<a00000010078d3e0>] wait_for_common+0x240/0x3c0
>                                 sp=e0000060b6dffd10 bsp=e0000060b6df1180
>  [<a00000010078d760>] wait_for_completion+0x40/0x60
>                                 sp=e0000060b6dffd40 bsp=e0000060b6df1160
>  [<a000000100114ee0>] __stop_machine_run+0x120/0x160
>                                 sp=e0000060b6dffd40 bsp=e0000060b6df1120
>  [<a000000100765ae0>] _cpu_down+0x2a0/0x600
>                                 sp=e0000060b6dffd80 bsp=e0000060b6df10c8
>  [<a000000100765ea0>] cpu_down+0x60/0xa0
>                                 sp=e0000060b6dffe20 bsp=e0000060b6df10a0
>  [<a000000100768090>] store_online+0x50/0xe0
>                                 sp=e0000060b6dffe20 bsp=e0000060b6df1070
>  [<a0000001004f8800>] sysdev_store+0x60/0xa0
>                                 sp=e0000060b6dffe20 bsp=e0000060b6df1038
>  [<a00000010022e370>] sysfs_write_file+0x250/0x300
>                                 sp=e0000060b6dffe20 bsp=e0000060b6df0fe0
>  [<a00000010018a750>] vfs_write+0x1b0/0x300
>                                 sp=e0000060b6dffe20 bsp=e0000060b6df0f90
>  [<a00000010018b350>] sys_write+0x70/0xe0
>                                 sp=e0000060b6dffe20 bsp=e0000060b6df0f18
>  [<a00000010000af80>] ia64_ret_from_syscall+0x0/0x20
>                                 sp=e0000060b6dffe30 bsp=e0000060b6df0f18
>  [<a000000000010720>] ia64_ivt+0xffffffff00010720/0x400
>                                 sp=e0000060b6e00000 bsp=e0000060b6df0f18

The following workaround alleviates the symptom and hopefully is a hint as to the solution:
	echo -1 > /proc/sys/kernel/sched_rt_runtime_us
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