Date: Tue, 20 Jun 2017 15:03:59 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: kan.liang@intel.com
Cc: linux-kernel@vger.kernel.org, dzickus@redhat.com, mingo@kernel.org,
        babu.moger@oracle.com, atomlin@redhat.com, prarit@redhat.com,
        torvalds@linux-foundation.org, peterz@infradead.org,
        tglx@linutronix.de, eranian@google.com, acme@redhat.com,
        ak@linux.intel.com, Kan Liang <Kan.liang@intel.com>,
        stable@vger.kernel.org
Subject: Re: [PATCH] kernel/watchdog: fix spurious hard lockups
Message-Id: <20170620150359.0fbb417aed72c84ac6ad8498@linux-foundation.org>
In-Reply-To: <20170620213309.30051-1-kan.liang@intel.com>
References: <20170620213309.30051-1-kan.liang@intel.com>
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On Tue, 20 Jun 2017 14:33:09 -0700 kan.liang@intel.com wrote:

> From: Kan Liang <Kan.liang@intel.com>
> 
> Some users reported spurious NMI watchdog timeouts.
> 
> We now have more and more systems where the Turbo range is wide enough
> that the NMI watchdog expires faster than the soft watchdog timer that
> updates the interrupt tick the NMI watchdog relies on.
> 
> This problem was originally added by commit 58687acba592
> ("lockup_detector: Combine nmi_watchdog and softlockup detector").
> Previously the NMI watchdog would always check jiffies, which were
> ticking fast enough. But now the backing is quite slow so the expire
> time becomes more sensitive.
> 
> For mainline the right fix is to switch the NMI watchdog to reference
> cycles, which tick always at the same rate independent of turbo mode.
> But this is requires some complicated changes in perf, which are too
> difficult to backport. Since we need a stable fix too just increase the
> NMI watchdog rate here to avoid the spurious timeouts. This is not an
> ideal fix because a 3x as large Turbo range could still fail, but for
> now that's not likely.
> 
> ...
>
> The right fix for mainline can be found here.
> perf/x86/intel: enable CPU ref_cycles for GP counter
> perf/x86/intel, watchdog: Switch NMI watchdog to ref cycles on x86
> https://patchwork.kernel.org/patch/9779087/
> https://patchwork.kernel.org/patch/9779089/

Presumably the "right fix" will later be altered to revert this
one-line workaround?