linux-kernel - Re: [PATCH 1/2] nohz: Affine unpinned timers to housekeepers

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Message-ID: <55E720DE.8020100@ezchip.com>
Date:	Wed, 2 Sep 2015 12:16:30 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>
CC:	"Jiang, Yunhong" <yunhong.jiang@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Vatika Harlalka <vatikaharlalka@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] nohz: Affine unpinned timers to housekeepers

On 09/02/2015 05:38 AM, Mike Galbraith wrote:
> IMHO, nohz_full -> cpu_isolated_map removal really wants to happen.
> NO_HZ_FULL_ALL currently means "Woohoo, next stop NR_CPUS=0".

Yeah, the problem seems to be folks who use it as a kind of
"hey, maybe this gives me some optimization boost somewhere"
kind of setting.  Did we ever hear actual use cases for people who
benefited from running nohz_full on cpus with an active scheduler,
i.e. no isolcpus for that core?  I find it hard to imagine, but, maybe...?

If we don't have such use cases, what should we do here?  I'm
slightly sympathetic to these folks who are going "Gee, my machine
suddenly got way slower", but only in the same sense as people
who shoot themselves in the foot and then say "Gee, my foot is
bleeding".  But maybe I'm being too hard core :-)

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

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