Re: [PATCH] rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config

[Neeraj Upadhyay <neeraju@codeaurora.org>]

Hi Paul,

On 9/25/2020 4:29 AM, Paul E. McKenney wrote:
> On Thu, Sep 24, 2020 at 12:04:10PM +0530, Neeraj Upadhyay wrote:
>> Clarify the "x" in rcuox/N naming in RCU_NOCB_CPU config
>> description.
>>
>> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> 
> Applied with a few additional updates as shown below.  As always, please
> let me know if I messed anything up.
> 

Looks good! thanks!


Thanks
Neeraj

> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 8d1d776b4998896a6f8f4608edb0b258bd37ec9f
> Author: Neeraj Upadhyay <neeraju@codeaurora.org>
> Date:   Thu Sep 24 12:04:10 2020 +0530
> 
>      rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config
>      
>      This commit clarifies that the "p" and the "s" in the in the RCU_NOCB_CPU
>      config-option description refer to the "x" in the "rcuox/N" kthread name.
>      
>      Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
>      [ paulmck: While in the area, update description and advice. ]
>      Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index b71e21f..cdc57b4 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -221,19 +221,23 @@ config RCU_NOCB_CPU
>   	  Use this option to reduce OS jitter for aggressive HPC or
>   	  real-time workloads.	It can also be used to offload RCU
>   	  callback invocation to energy-efficient CPUs in battery-powered
> -	  asymmetric multiprocessors.
> +	  asymmetric multiprocessors.  The price of this reduced jitter
> +	  is that the overhead of call_rcu() increases and that some
> +	  workloads will incur significant increases in context-switch
> +	  rates.
>   
>   	  This option offloads callback invocation from the set of CPUs
>   	  specified at boot time by the rcu_nocbs parameter.  For each
>   	  such CPU, a kthread ("rcuox/N") will be created to invoke
>   	  callbacks, where the "N" is the CPU being offloaded, and where
> -	  the "p" for RCU-preempt (PREEMPTION kernels) and "s" for RCU-sched
> -	  (!PREEMPTION kernels).  Nothing prevents this kthread from running
> -	  on the specified CPUs, but (1) the kthreads may be preempted
> -	  between each callback, and (2) affinity or cgroups can be used
> -	  to force the kthreads to run on whatever set of CPUs is desired.
> -
> -	  Say Y here if you want to help to debug reduced OS jitter.
> +	  the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
> +	  RCU-sched (!PREEMPTION kernels).  Nothing prevents this kthread
> +	  from running on the specified CPUs, but (1) the kthreads may be
> +	  preempted between each callback, and (2) affinity or cgroups can
> +	  be used to force the kthreads to run on whatever set of CPUs is
> +	  desired.
> +
> +	  Say Y here if you need reduced OS jitter, despite added overhead.
>   	  Say N here if you are unsure.
>   
>   config TASKS_TRACE_RCU_READ_MB
> 

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