linux-kernel - Re: [PATCH] rcu: Report a quiescent state of TASKS

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Date:   Mon, 14 May 2018 21:11:11 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     jiangshanlai@...il.com, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com, joel@...lfernandes.org
Subject: Re: [PATCH] rcu: Report a quiescent state of TASKS_RCU on a tick
 from user

On Tue, May 15, 2018 at 09:33:46AM +0900, Byungchul Park wrote:
> Hello Paul,
> 
> You removed the reporing while simplifying the commit 508880df6 :)
> Fold this patch onto the commit or add, whatever you want.

First, thank you for checking!

But second, the removal was intentional.  Tiny RCU only exists in
PREEMPT=n kernels, and in such kernels there can be no RCU-tasks.
This is the reason for this line in the new commit log:

[ paulmck: Simplify rcutiny portion given no RCU-tasks for !PREEMPT. ]

							Thanx, Paul

> Thanks,
> Byungchul
> 
> ----->8-----
> >From 18a2d8da3baf79d0edd5ccf94abe6f989da5b1c1 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@....com>
> Date: Tue, 15 May 2018 09:21:43 +0900
> Subject: [PATCH] rcu: Report a quiescent state of TASKS_RCU on a tick from
>  user
> 
> The reporting was removed while simplifying the commit 508880df6 (rcu:
> Improve rcu_note_voluntary_context_switch() reporting). Add it back.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/rcu/tiny.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index befc932..3345596 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -120,8 +120,10 @@ void rcu_bh_qs(void)
>   */
>  void rcu_check_callbacks(int user)
>  {
> -	if (user)
> +	if (user) {
>  		rcu_sched_qs();
> +		rcu_tasks_qs();
> +	}
>  	if (user || !in_softirq())
>  		rcu_bh_qs();
>  }
> -- 
> 1.9.1
>