linux-kernel - Re: [RFC GIT PULL] nohz: Kconfig layout improvements

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Message-ID: <CAFTL4hyToYPCsKUrQmtUxeN+X7=40vS9vnB0_nk8X+6-vm_KSQ@mail.gmail.com>
Date:	Wed, 10 Apr 2013 15:47:33 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Marek <mmarek@...e.cz>
Subject: Re: [RFC GIT PULL] nohz: Kconfig layout improvements

2013/4/4 Christoph Lameter <cl@...ux.com>:
> It seems that nohz still has no effect.
>
> 3.9-rc5 + patches. Affinity of init set to 0,1 so no
> tasks are running on 9. The "latencytest" used here is part of my
> lldiag-0.15 toolkit.
>
> First test without any special kernel parameters. nohz off right?
>
> $ nice -5 taskset -c 9 latencytest
>
> CPUs: Freq=2.90Ghz Processors=32 Cores=8 cacheline_size=64 Intel(R)
> Xeon(R) CPU E5-2690 0 @ 2.90GHz
> 16775106 samples below 1000 nsec
> 13 involuntary context switches
> 1019 (0.00607411%) variances in 10.00 seconds: minimum 1.07us maximum 12.32us average 3.30us stddev 0.63us
>
> HZ=100 so the 1019 variances are likely timer interrupts.
>
>
>
>
> After nohz setup
>
> /proc/cmdline:
>
> BOOT_IMAGE=/vmlinuz-3.9.0-rc5+ root=/dev/mapper/vg01-root ro console=tty0 console=ttyS0,115200 idle=mwait rcu_nocb_poll rcu_nocbs=2-31 nohz_extended=2-31
>
> $ nice -5 taskset -c 9 latencytest
> CPUs: Freq=2.90Ghz Processors=32 Cores=8 cacheline_size=64 Intel(R)
> Xeon(R) CPU E5-2690 0 @ 2.90GHz
> 16779362 samples below 1000 nsec
> 13 involuntary context switches
> 1037 (0.00617983%) variances in 10.00 seconds: minimum 1.00us maximum 10.61us average 3.30us stddev 0.98us
>
>
>
> If I move the RCU threads off the cpu then I get a slightly better result:
>
> $ nice -5 taskset -c 9 latencytest
> CPUs: Freq=2.90Ghz Processors=32 Cores=8 cacheline_size=64 Intel(R)
> Xeon(R) CPU E5-2690 0 @ 2.90GHz
> 16796039 samples below 1000 nsec
> 12 involuntary context switches
> 1020 (0.00607249%) variances in 10.00 seconds: minimum 1.00us maximum 11.58us average 2.77us stddev 0.55us
>
>
>
> Why is the tick not stopping? How do I diagnose this? (I can start
> patching the kernel again like last time but isnt there a better way?)

I don't know which tree you are using. But if you have that patch in:
http://git.kernel.org/cgit/linux/kernel/git/frederic/linux-dynticks.git/commit/?h=3.9-rc1-nohz1&id=451128553e5e827dccc6cbcd24238470ec693d90
looking at the traces on that CPU may give you a few hints. Then you
can dig deeper by looking at the sched_switch, timers, irq, ... events
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