linux-kernel - Re: [PATCH V2 3/7] rcu: remove useless special.b.deferred

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Message-ID: <20200217232353.GB17570@paulmck-ThinkPad-P72>
Date:   Mon, 17 Feb 2020 15:23:53 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH V2 3/7] rcu: remove useless special.b.deferred_qs

On Sat, Nov 02, 2019 at 12:45:55PM +0000, Lai Jiangshan wrote:
> ->read_read_unlock_special.b.deferred_qs is set when
> ->read_read_unlock_special is non-zero, and it is cleared when
> ->read_read_unlock_special is cleared.
> 
> So it is useless.
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>

I have queued this on -rcu for further review and testing, thank you!

							Thanx, Paul

> ---
>  include/linux/sched.h    | 2 +-
>  kernel/rcu/tree_plugin.h | 1 -
>  2 files changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2c2e56bd8913..3ba392d71de9 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -604,7 +604,7 @@ union rcu_special {
>  		u8			blocked;
>  		u8			need_qs;
>  		u8			exp_hint; /* Hint for performance. */
> -		u8			deferred_qs;
> +		u8			pad; /* No garbage from compiler! */
>  	} b; /* Bits. */
>  	u32 s; /* Set of bits. */
>  };
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 2fab8be2061f..f2fd7d687bdb 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -622,7 +622,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
>  			}
>  		}
> -		t->rcu_read_unlock_special.b.deferred_qs = true;
>  		local_irq_restore(flags);
>  		return;
>  	}
> -- 
> 2.20.1
>