Re: [PATCH 6/6] sched/isolation: Tick offload documentation

[Ingo Molnar]

* Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:

> Update the documentation to reflect the 1Hz tick offload changes.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
> Cc: Mike Galbraith <efault@xxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Wanpeng Li <kernellwp@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 39ac9d4..c851e41 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1762,7 +1762,11 @@
>  			specified in the flag list (default: domain):
>  
>  			nohz
> -			  Disable the tick when a single task runs.
> +			  Disable the tick when a single task runs. A residual 1Hz
> +			  tick is offloaded to workqueues that you need to affine
> +			  to housekeeping through the sysfs file
> +			  /sys/devices/virtual/workqueue/cpumask or using the below
> +			  domain flag.

This is pretty ambiguous and somewhat confusing, I'd suggest something like:

			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.

Assuming what I wrote is correct - the CPU isolation config space is pretty 
confusing all around and should be made a lot more human friendly ...

Thanks,

	Ingo