Date: Tue, 12 May 2015 14:34:40 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>, Steven Rostedt <rostedt@goodmis.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@linux.vnet.ibm.com,
        Gilad Ben Yossef <giladb@ezchip.com>, Rik van Riel <riel@redhat.com>,
        Tejun Heo <tj@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Christoph Lameter <cl@linux.com>,
        "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
        linux-doc@vger.kernel.org, linux-api@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: CONFIG_ISOLATION=y (was: [PATCH 0/6] support "dataplane" mode
 for nohz_full)
Message-ID: <20150512123440.GA16959@gmail.com>
References: <20150508161909.308d60e21f6b83b897174276@linux-foundation.org>
 <20150509070538.GA9413@gmail.com>
 <20150511085759.71deeb64@gandalf.local.home>
 <20150511171916.GN6776@linux.vnet.ibm.com>
 <20150511102744.9ebb2d05a7e8b457d03430bf@linux-foundation.org>
 <20150511173305.GC32512@lerouge>
 <20150511140009.1f7bcf07@gandalf.local.home>
 <5550F077.6030906@ezchip.com>
 <20150512091032.GA10138@gmail.com>
 <20150512114809.GL21418@twins.programming.kicks-ass.net>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, May 12, 2015 at 11:10:32AM +0200, Ingo Molnar wrote:
> > 
> > So I'd vote for Frederic's CONFIG_ISOLATION=y, mostly because this 
> > is a high level kernel feature, so it won't conflict with 
> > isolation concepts in lower level subsystems such as IOMMU 
> > isolation - and other higher level features like scheduler 
> > isolation are basically another partial implementation we want to 
> > merge with all this...
> 
> But why do we need a CONFIG flag for something that has no content?
> 
> That is, I do not see anything much; except the 'I want to stay in 
> userspace and kill me otherwise' flag, and I'm not sure that 
> warrants a CONFIG flag like this.
> 
> Other than that, its all a combination of NOHZ_FULL and 
> cpusets/isolcpus and whatnot.

Yes, that's what I meant: CONFIG_ISOLATION would trigger what is 
NO_HZ_FULL today - we could possibly even remove CONFIG_NO_HZ_FULL as 
an individual Kconfig option?

CONFIG_ISOLATION=y would express the guarantee from the kernel that 
it's possible for user-space to configure itself to run undisturbed - 
instead of the current inconsistent set of options and facilities.

A bit like CONFIG_PREEMPT_RT is more than just preemptable spinlocks, 
it also tries to offer various facilities and tune the defaults to 
turn the kernel hard-rt.

Does that make sense to you?

Thanks,

	Ingo

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