linux-kernel - Re: [PATCH v2 2/2] nohz: make nohz

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Message-ID: <55266A54.30304@ezchip.com>
Date:	Thu, 9 Apr 2015 08:02:28 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] nohz: make nohz_full imply isolcpus

On 4/9/2015 4:29 AM, Peter Zijlstra wrote:
> On Wed, Apr 08, 2015 at 02:12:34PM -0400, Chris Metcalf wrote:
>
>>> 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.
> No its not, you should correct me when I'm wrong ;-)

Oh, I have no problem with that :-)  But, now that I know what was confusing
you about the patch I see what it was you were saying with your English
above too.  I thought you were saying something like "making nohz_full
imply isolcpus" again, but you weren't.  Phew, OK, I think we're done
talking at cross-purposes.

> So the problem is that:
>
> +       tick_nohz_full_set_cpus(cpu_isolated_map);
>
> reads like you're doing:
>
>    nohz_full_map |= isolcpus_map
>
> But in actual fact you're doing:
>
>    isolcpus_map |= nohz_full_map
>
> So that function is retarded, but the logic is fine.
>
> So NAK on both patches for the reason that they're utterly confusing as
> to wtf they actually do.

What about tick_nohz_full_cpumask_or(cpu_isolated_map) ?
At that point maybe the similarity to the existing cpumask API will make
it more clear that we are modifying the argument?

If not, do you have any suggestions what might do better?  Obviously
the goal is to make it something that macroizes away, otherwise I'd
suggest just explicitly using an #ifdef and cpumask_or().

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

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