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Date: Mon, 23 Apr 2007 23:49:10 -0700 From: Andrew Morton <akpm@...ux-foundation.org> To: Jeremy Fitzhardinge <jeremy@...p.org> Cc: Ingo Molnar <mingo@...e.hu>, Linux Kernel <linux-kernel@...r.kernel.org>, virtualization@...ts.osdl.org, Prarit Bhargava <prarit@...hat.com>, Eric Dumazet <dada1@...mosbay.com>, Thomas Gleixner <tglx@...utronix.de>, john stultz <johnstul@...ibm.com>, Zachary Amsden <zach@...are.com>, James Morris <jmorris@...ei.org>, Dan Hecht <dhecht@...are.com>, Paul Mackerras <paulus@...ba.org>, Martin Schwidefsky <schwidefsky@...ibm.com>, Chris Lalancette <clalance@...hat.com>, Rick Lindsley <ricklind@...ibm.com> Subject: Re: [patch 1/4] Ignore stolen time in the softlockup watchdog On Tue, 27 Mar 2007 14:49:20 -0700 Jeremy Fitzhardinge <jeremy@...p.org> wrote: > The softlockup watchdog is currently a nuisance in a virtual machine, > since the whole system could have the CPU stolen from it for a long > period of time. While it would be unlikely for a guest domain to be > denied timer interrupts for over 10s, it could happen and any softlockup > message would be completely spurious. > > Earlier I proposed that sched_clock() return time in unstolen > nanoseconds, which is how Xen and VMI currently implement it. If the > softlockup watchdog uses sched_clock() to measure time, it would > automatically ignore stolen time, and therefore only report when the > guest itself locked up. When running native, sched_clock() returns > real-time nanoseconds, so the behaviour would be unchanged. > > Note that sched_clock() used this way is inherently per-cpu, so this > patch makes sure that the per-processor watchdog thread initialized > its own timestamp. This patch (ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.21-rc6/2.6.21-rc6-mm1/broken-out/ignore-stolen-time-in-the-softlockup-watchdog.patch) causes six failures in the locking self-tests, which I must say is rather clever of it. Here's the first one: [17179569.184000] Lock dependency validator: Copyright (c) 2006 Red Hat, Inc., Ingo Molnar [17179569.184000] ... MAX_LOCKDEP_SUBCLASSES: 8 [17179569.184000] ... MAX_LOCK_DEPTH: 30 [17179569.184000] ... MAX_LOCKDEP_KEYS: 2048 [17179569.184000] ... CLASSHASH_SIZE: 1024 [17179569.184000] ... MAX_LOCKDEP_ENTRIES: 8192 [17179569.184000] ... MAX_LOCKDEP_CHAINS: 16384 [17179569.184000] ... CHAINHASH_SIZE: 8192 [17179569.184000] memory used by lock dependency info: 992 kB [17179569.184000] per task-struct memory footprint: 1200 bytes [17179569.184000] ------------------------ [17179569.184000] | Locking API testsuite: [17179569.184000] ---------------------------------------------------------------------------- [17179569.184000] | spin |wlock |rlock |mutex | wsem | rsem | [17179569.184000] -------------------------------------------------------------------------- [17179569.184000] A-A deadlock: ok | ok | ok | ok | ok | ok | [17179569.184000] A-B-B-A deadlock: ok | ok | ok | ok | ok | ok | [17179569.184000] A-B-B-C-C-A deadlock: ok | ok | ok | ok | ok | ok | [17179569.184001] A-B-C-A-B-C deadlock: ok | ok | ok | ok | ok | ok | [17179569.184002] A-B-B-C-C-D-D-A deadlock: ok | ok | ok | ok | ok | ok | [17179569.184003] A-B-C-D-B-D-D-A deadlock: ok | ok | ok | ok | ok | ok | [17179569.184004] A-B-C-D-B-C-D-A deadlock: ok | ok | ok | ok | ok | ok | [17179569.184005] double unlock: ok | ok | ok | ok | ok | ok | [17179569.184006] initialize held: ok | ok | ok | ok | ok | ok | [17179569.184006] bad unlock order: ok | ok | ok | ok | ok | ok | [17179569.184006] -------------------------------------------------------------------------- [17179569.184006] recursive read-lock: | ok | | ok | [17179569.184006] recursive read-lock #2: | ok | | ok | [17179569.184007] mixed read-write-lock: | ok | | ok | [17179569.184007] mixed write-read-lock: | ok | | ok | [17179569.184007] -------------------------------------------------------------------------- [17179569.184007] hard-irqs-on + irq-safe-A/12: ok | ok | ok | [17179569.184007] soft-irqs-on + irq-safe-A/12: ok | ok | ok | [17179569.184007] hard-irqs-on + irq-safe-A/21: ok | ok | ok | [17179569.184007] soft-irqs-on + irq-safe-A/21: ok | ok | ok | [17179569.184007] sirq-safe-A => hirqs-on/12: ok | ok |irq event stamp: 458 [17179569.184007] hardirqs last enabled at (458): [<c01e4116>] irqsafe2A_rlock_12+0x96/0xa3 [17179569.184007] hardirqs last disabled at (457): [<c01095b9>] sched_clock+0x5e/0xe9 [17179569.184007] softirqs last enabled at (454): [<c01e4101>] irqsafe2A_rlock_12+0x81/0xa3 [17179569.184007] softirqs last disabled at (450): [<c01e408b>] irqsafe2A_rlock_12+0xb/0xa3 [17179569.184007] FAILED| [<c0104cf0>] dump_trace+0x63/0x1ec [17179569.184007] [<c0104e93>] show_trace_log_lvl+0x1a/0x30 [17179569.184007] [<c01059ec>] show_trace+0x12/0x14 [17179569.184007] [<c0105a45>] dump_stack+0x16/0x18 [17179569.184007] [<c01e1eb5>] dotest+0x6b/0x3d0 [17179569.184007] [<c01eb249>] locking_selftest+0x915/0x1a58 [17179569.184007] [<c048c979>] start_kernel+0x1d0/0x2a2 [17179569.184007] ======================= [17179569.184007] [17179569.184007] sirq-safe-A => hirqs-on/21:irq event stamp: 462 [17179569.184007] hardirqs last enabled at (462): [<c01e3eb2>] irqsafe2A_spin_21+0x1b/0xa3 [17179569.184007] hardirqs last disabled at (461): [<c01095b9>] sched_clock+0x5e/0xe9 [17179569.184007] softirqs last enabled at (454): [<c01e4101>] irqsafe2A_rlock_12+0x81/0xa3 [17179569.184007] softirqs last disabled at (450): [<c01e408b>] irqsafe2A_rlock_12+0xb/0xa3 [17179569.184007] ok |irq event stamp: 466 It's a challenge to even find the code which corresponds with this failure, so good luck. It seems fairly sensitive to .config settings. See http://userweb.kernel.org/~akpm/config-sony.txt Unfortunately this causes lockdep to disable itself, thus hiding all the other bugs which people have contributed, so I'll drop your patch for now. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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