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Date: Tue, 20 Oct 2015 23:03:33 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Gilad Ben Yossef <giladb@ezchip.com>, Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Tejun Heo <tj@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 08/14] nohz_full: allow disabling the 1Hz minimum tick
 at boot
Message-ID: <20151020210331.GC29289@lerouge>
References: <1445373372-6567-1-git-send-email-cmetcalf@ezchip.com>
 <1445373372-6567-9-git-send-email-cmetcalf@ezchip.com>
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On Tue, Oct 20, 2015 at 04:36:06PM -0400, Chris Metcalf wrote:
> While the current fallback to 1-second tick is still required for
> a number of kernel accounting tasks (e.g. vruntime, load balancing
> data, and load accounting), it's useful to be able to disable it
> for testing purposes.  Paul McKenney observed that if we provide
> a mode where the 1Hz fallback timer is removed, this will provide
> an environment where new code that relies on that tick will get
> punished, and we won't forgive such assumptions silently.
> 
> This option also allows easy testing of nohz_full and task-isolation
> modes to determine what functionality needs to be implemented,
> and what possibly-spurious timer interrupts are scheduled when
> the basic 1Hz tick has been turned off.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>

There have been proposals to disable/tune the 1 Hz tick via debugfs which
I Nacked because once you give such an opportunity to the users, they
will use that hack and never fix the real underlying issue.

For the same reasons, I'm sorry but I have to Nack this proposal as well.

If this is for development or testing purpose, scheduler_max_tick_deferment() is
easily commented out.