linux-kernel - Re: [ANNOUNCE] 3.8-rc6-nohz4

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Message-ID: <CAFTL4hySiyn8Ci6XKFSRsMr7FabgpwSbOuNDP_GsefFDsbT2Cg@mail.gmail.com>
Date:	Thu, 7 Feb 2013 17:25:51 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [ANNOUNCE] 3.8-rc6-nohz4

2013/2/7 Ingo Molnar <mingo@...nel.org>:
>
> * Steven Rostedt <rostedt@...dmis.org> wrote:
>
>> I'll reply to this as I come up with comments.
>>
>> First thing is, don't call it NO_HZ_FULL. A better name would
>> be NO_HZ_CPU. I would like to reserve NO_HZ_FULL when we
>> totally remove jiffies :-)
>
> I don't think we want yet another config option named in a
> weird way.
>
> What we want instead is to just split NO_HZ up into its
> conceptual parts:
>
>    CONFIG_NO_HZ_IDLE

Renaming CONFIG_NO_HZ to CONFIG_NO_HZ_IDLE is something I considered.
I was just worried about this option being present in many defconfig.
Perhaps we can do that renaming and keep CONFIG_NO_HZ around a little
while for backward compatibility (pretty much like what we've done for
CONFIG_PERF_COUNTERS -> CONFIG_PERF_EVENTS).

>    CONFIG_NO_HZ_USER_SPACE
>    CONFIG_NO_HZ_KERNEL_SPACE
>
> Where the current status quo is NO_HZ_IDLE=y, and Frederic is
> about to introduce NO_HZ_USER_SPACE=y. When jiffies get removed
> we get NO_HZ_KERNEL_SPACE=y.

Note on my tree I stop the tick on both rings. I believe that
restarting the tick on kernel entry isn't something we should
seriously consider. It would be a costly operation that may make
things worse. And in fact there is no big difference. Just kernelspace
has more opportunities to be disturbed (RCU IPIs, async timer/work
scheduled by the kernel, etc...) and get its tick restarted sometimes.

>
> The 'CONFIG_NO_HZ' meta-option, which we should leave for easy
> configurability and for compatibility, should get us the
> currently recommended default, which for the time being might
> be:

Ah looks like you considered the compatibility as well :)

>
>    CONFIG_NO_HZ_IDLE=y
>    # CONFIG_NO_HZ_USER_SPACE is disabled
>
> Btw., you could add CONFIG_NO_HZ_KERNEL_SPACE right away, just
> keep it false all the time. That would document our future plans
> pretty well.
>
> Once CONFIG_NO_HZ_USER_SPACE is proven problem-free, we might
> default to:
>
>    CONFIG_NO_HZ_IDLE=y
>    CONFIG_NO_HZ_USER_SPACE=y
>
> The goal is to have this in the distant future:
>
>    CONFIG_NO_HZ=y
>
>    CONFIG_NO_HZ_IDLE=y
>    CONFIG_NO_HZ_USER_SPACE=y
>    CONFIG_NO_HZ_KERNEL_SPACE=y
>
> And eventually we might even be able to get rid of all the 3
> variants, and only offer full-on/off.
>
> Agreed?

At least for now we seem to agree on CONFIG_NO_HZ_IDLE and keep
CONFIG_NO_HZ for compatibility. Are you ok with that? If so I'll send
a patch.
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