linux-kernel - Re: [PATCH 6/6] sched/isolation: Update nohz documentation to explain tick offload

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Date:   Sat, 17 Feb 2018 11:53:22 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 6/6] sched/isolation: Update nohz documentation to explain
 tick offload

On Thu, 15 Feb 2018, Frederic Weisbecker wrote:

> Update the documentation to reflect the 1Hz tick offload changes.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Chris Metcalf <cmetcalf@...lanox.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Luiz Capitulino <lcapitulino@...hat.com>
> Cc: Mike Galbraith <efault@....de>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Wanpeng Li <kernellwp@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 1d1d53f..50b9837 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1766,6 +1766,17 @@
>  
>  			nohz
>  			  Disable the tick when a single task runs.
> +
> +			  A residual 1Hz tick is offloaded to workqueues, which you
> +			  need to affine to housekeeping through the global
> +			  workqueue's affinity configured via the
> +			  /sys/devices/virtual/workqueue/cpumask sysfs file, or
> +			  by using the 'domain' flag described below.
> +
> +			  NOTE: by default the global workqueue runs on all CPUs,
> +			  so to protect individual CPUs the 'cpumask' file has to
> +			  be configured manually after bootup.

Hmm. Why don't you set that to the housekeeping cpus by default?

Thanks,

	tglx