Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

[Peter Zijlstra]

On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote:
> If you set RCU_FANOUT_LEAF too high, you can get lock contention
> on the leaf rcu_node, and you should boot with the skew_tick kernel
> parameter set in order to avoid this lock contention.  This commit
> therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state
> this.
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  init/Kconfig | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index a92f27da4a27..946e561e67b7 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF
>  	  initialization.  These systems tend to run CPU-bound, and thus
>  	  are not helped by synchronized interrupts, and thus tend to
>  	  skew them, which reduces lock contention enough that large
> -	  leaf-level fanouts work well.
> +	  leaf-level fanouts work well.  That said, setting leaf-level
> +	  fanout to a large number will likely cause problematic
> +	  lock contention on the leaf-level rcu_node structures unless
> +	  you boot with the skew_tick kernel parameter.

Why mention a way out of a problem you shouldn't have to begin with?

Just state its bad and result in lock contention and leave it at that.