linux-kernel - Re: [PATCH v8 08/14] nohz_full: allow disabling the 1Hz minimum tick at boot

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Date:	Tue, 20 Oct 2015 17:18:13 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andy Lutomirski <luto@...capital.net>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 08/14] nohz_full: allow disabling the 1Hz minimum tick
 at boot

On 10/20/2015 05:03 PM, Frederic Weisbecker wrote:
> 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@...hip.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.

Fair enough and certainly your prerogative, so don't hesitate to
say "no" to the following argument.  :-)

I would tend to differentiate a debugfs proposal from a boot flag
proposal: a boot flag is a more hardcore thing to change, and it's
not like application developers will come along and explain that
you have to boot with different flags to run their app - whereas
if they can just sneak in a modification to a debugfs setting that's
much easier for the app to tweak.

So perhaps a boot flag is an acceptable compromise between
"nothing" and a debugfs tweak?  It certainly does make it easier
to hack on the task-isolation code, and likely other things where
people are trying out fixes to subsystems where they are attempting
to remove the reliance on the tick.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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