linux-kernel - Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks range

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Message-ID: <0000013def390189-5508c590-e421-49e7-9eab-a3924489091a-000000@email.amazonses.com>
Date:	Tue, 9 Apr 2013 14:35:23 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
cc:	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks
 range

On Tue, 9 Apr 2013, Paul Gortmaker wrote:

> > 2. Avoid the setting of cpus entirely? If full nohz mode is desired
> > then pick one cpu (f.e. the first one or the one that is used for xtime
> > updates) and then make all other cpus nohz. Set the affinity mask for the
> > rcuoXXX threads to that cpu.
>
> I can imagine people with multi socket systems wanting to have
> a system partitioned with one "normal" core per physical socket,
> for timekeeping, RCU threads, etc, but #2 would prevent that.

That is a good point. The kernel needs to run per node threads for I/O and
reclaim which could have their home there. Just had some bad experience
with latency introduced by the block layer redirecting I/O to the first
processor of a node. Maybe we can adopt that convention and relocate
kernel processing as much as possible to the first processor of a
node?


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