Message-ID: <55256F92.1010606@ezchip.com>
Date: Wed, 8 Apr 2015 14:12:34 -0400
From: Chris Metcalf <cmetcalf@ezchip.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Frederic Weisbecker <fweisbec@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 2/2] nohz: make nohz_full imply isolcpus
References: <5522BF1E.2040606@ezchip.com> <1428344205-27678-1-git-send-email-cmetcalf@ezchip.com> <1428344205-27678-2-git-send-email-cmetcalf@ezchip.com> <20150408094114.GX23123@twins.programming.kicks-ass.net> <5525357B.2080205@ezchip.com> <20150408142641.GG5029@twins.programming.kicks-ass.net> <55254794.4070503@ezchip.com> <20150408172746.GK5029@twins.programming.kicks-ass.net>
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On 04/08/2015 01:27 PM, Peter Zijlstra wrote:
> On Wed, Apr 08, 2015 at 11:21:56AM -0400, Chris Metcalf wrote:
>> Apparently the body of the commit message isn't as clear as it might be :-)
>>
>> It does say the same thing, though, basically that if nohz_full DOESN'T
>> imply isolcpus, that's a bad thing.  I'm happy to reword the text to avoid
>> the double negative and say:
>>
>>    nohz_full is only useful with isolcpus also set, since otherwise the
>>    scheduler has to run periodically to try to determine whether to steal
>>    work from other cores.
> But you're doing the reverse! You're setting nohz_full for isolcpus, not
> limiting the nohz_full mask to isolcpus.

Ah, I see.  Yes, that's right.  The idea is that if you are saying
"nohz_full=1-15" on the command line, you would like that to
imply "isolcpus=1-15" as well, without having to actually say so
explicitly.  If we instead limit nohz_full based on isolcpus, it's not
clear that it's actually worth making any change in this area.

I still maintain that the text has always correctly (if perhaps
confusingly) said what it is that the code was doing; where the
text says "x implies y", that means "x being set forces y to be set".
But I'm respinning it anyway for Frederic so I will avoid using the
word "imply" altogether to make this clearer.

The larger question is whether you agree with the proposed semantics.

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

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