From: Thomas Gleixner <tglx@linutronix.de>
To: Waiman Long <longman@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org, Mike Rapoport <rppt@linux.ibm.com>,
Kees Cook <keescook@chromium.org>,
Waiman Long <longman@redhat.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Robert Richter <rrichter@marvell.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup
Date: Thu, 16 Jan 2020 12:44:07 +0100 [thread overview]
Message-ID: <87blr3wrqw.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <87sgkgw3xq.fsf@nanos.tec.linutronix.de>
Thomas Gleixner <tglx@linutronix.de> writes:
Added ARM64 and ThunderX folks
> Waiman Long <longman@redhat.com> writes:
>> By adding some instrumentation code, it was found that for cpu 14,
>> watchdog_enable() was called early with a timestamp of 1. That activates
>> the watchdog time checking logic. It was also found that the monotonic
>> time measured during the smp_init() phase runs much slower than the
>> real elapsed time as shown by the below debug printf output:
>>
>> [ 1.138522] run_queues, watchdog_timer_fn: now = 170000000
>> [ 25.519391] run_queues, watchdog_timer_fn: now = 4170000000
>>
>> In this particular case, it took about 24.4s of elapsed time for the
>> clock to advance 4s which is the soft expiration time that is required
>> to trigger the calling of watchdog_timer_fn(). That clock slowdown
>> stopped once the smp_init() call was done and the clock time ran at
>> the same rate as the elapsed time afterward.
And looking at this with a more awake brain, the root cause is pretty
obvious.
sched_clock() advances by 24 seconds, but clock MONOTONIC on which the
watchdog timer is based does not. As the timestamps you printed have 7
trailing zeros, it's pretty clear that timekeeping is still jiffies
based at this point and HZ is set to 100.
So while bringing up the non-boot CPUs the boot CPU loses ~2000 timer
interrupts. That needs to be fixed and not papered over.
Thanks,
tglx
next prev parent reply other threads:[~2020-01-16 11:44 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-03 15:10 [PATCH v2] watchdog: Fix possible soft lockup warning at bootup Waiman Long
2020-01-16 2:06 ` Thomas Gleixner
2020-01-16 11:44 ` Thomas Gleixner [this message]
2020-01-16 15:11 ` Robert Richter
2020-01-16 16:57 ` Thomas Gleixner
2020-01-16 17:34 ` Waiman Long
2020-01-16 19:10 ` Thomas Gleixner
2020-01-16 19:13 ` Waiman Long
2020-01-16 18:17 ` [PATCH] watchdog/softlockup: Enforce that timestamp is valid on boot Thomas Gleixner
2020-01-17 10:25 ` [tip: core/core] " tip-bot2 for Thomas Gleixner
2020-01-16 15:34 ` [PATCH v2] watchdog: Fix possible soft lockup warning at bootup Waiman Long
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