Re: [PATCH] rcu: When rcuog kthreads is in polling mode, wakeup waitqueue is not requried

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Jan 28, 2022 at 11:13:46AM +0800, Zqiang wrote:
> When grace period cleanup, the rcuog kthreads that waiting in sq
> waitqueue will be awakened, however if the 'rcu_nocb_poll' is set,
> the sq waitqueue always empty, so if 'rcu_nocb_poll' is set, return
> directly.

This does decrease grace-period-cleanup overhead in kernels built with
CONFIG_RCU_NOCB_CPU=y and booted with the rcu_nocb_poll kernel boot
parameter set.  On the other hand, it increases grace-period-cleanup
overhead in kernels built with CONFIG_RCU_NOCB_CPU=y but booted without
the rcu_nocb_poll kernel boot parameter set.

Last I checked, more kernels were booted without the rcu_nocb_poll kernel
boot parameter set.  If this is still the case, this patch would slow
things down for most systems.

Or are there now lots of systems booted with rcu_nocb_poll?

							Thanx, Paul

> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
>  kernel/rcu/tree_nocb.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 636d0546a4e9..9e106c590e56 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -201,6 +201,8 @@ static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
>   */
>  static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
>  {
> +	if (rcu_nocb_poll)
> +		return;
>  	swake_up_all(sq);
>  }
>  
> -- 
> 2.25.1
>