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Date:   Wed, 29 Aug 2018 21:56:16 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH tip/core/rcu 3/3] srcu: Make early-boot call_srcu()
 reuse workqueue lists

On Wed, 29 Aug 2018 14:23:13 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> Allocating a list_head structure that is almost never used, and, when
> used, is used only during early boot (rcu_init() and earlier), is a bit
> wasteful.  This commit therefore eliminates that list_head in favor of
> the one in the work_struct structure.  This is safe because the work_struct
> structure cannot be used until after rcu_init() returns.
> 
> Reported-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> ---
>  include/linux/srcutiny.h |  2 --
>  include/linux/srcutree.h |  3 +--
>  kernel/rcu/srcutiny.c    | 10 +++++-----
>  kernel/rcu/srcutree.c    | 11 +++++------
>  4 files changed, 11 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> index 2b5c0822e683..f41d2fb09f87 100644
> --- a/include/linux/srcutiny.h
> +++ b/include/linux/srcutiny.h
> @@ -36,7 +36,6 @@ struct srcu_struct {
>  	struct rcu_head *srcu_cb_head;	/* Pending callbacks: Head. */
>  	struct rcu_head **srcu_cb_tail;	/* Pending callbacks: Tail. */
>  	struct work_struct srcu_work;	/* For driving grace periods. */
> -	struct list_head srcu_boot_entry; /* Early-boot callbacks. */
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map dep_map;
>  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> @@ -49,7 +48,6 @@ void srcu_drive_gp(struct work_struct *wp);
>  	.srcu_wq = __SWAIT_QUEUE_HEAD_INITIALIZER(name.srcu_wq),	\
>  	.srcu_cb_tail = &name.srcu_cb_head,				\
>  	.srcu_work = __WORK_INITIALIZER(name.srcu_work, srcu_drive_gp),	\
> -	.srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry),	\
>  	__SRCU_DEP_MAP_INIT(name)					\
>  }
>  
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index 9cfa4610113a..0ae91b3a7406 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -94,7 +94,6 @@ struct srcu_struct {
>  						/*  callback for the barrier */
>  						/*  operation. */
>  	struct delayed_work work;
> -	struct list_head srcu_boot_entry;	/* Early-boot callbacks. */
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map dep_map;
>  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> @@ -110,7 +109,7 @@ struct srcu_struct {
>  	.sda = &pcpu_name,						\
>  	.lock = __SPIN_LOCK_UNLOCKED(name.lock),			\
>  	.srcu_gp_seq_needed = -1UL,					\
> -	.srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry),	\
> +	.work = __DELAYED_WORK_INITIALIZER(name.work, NULL, 0),		\

Thanks!

-- Steve

>  	__SRCU_DEP_MAP_INIT(name)					\
>  }
>  
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index d233f0c63f6f..b46e6683f8c9 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -48,7 +48,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp)
>  	sp->srcu_gp_waiting = false;
>  	sp->srcu_idx = 0;
>  	INIT_WORK(&sp->srcu_work, srcu_drive_gp);
> -	INIT_LIST_HEAD(&sp->srcu_boot_entry);
> +	INIT_LIST_HEAD(&sp->srcu_work.entry);
>  	return 0;
>  }
>  
> @@ -185,8 +185,8 @@ void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
>  	if (!READ_ONCE(sp->srcu_gp_running)) {
>  		if (likely(srcu_init_done))
>  			schedule_work(&sp->srcu_work);
> -		else if (list_empty(&sp->srcu_boot_entry))
> -			list_add(&sp->srcu_boot_entry, &srcu_boot_list);
> +		else if (list_empty(&sp->srcu_work.entry))
> +			list_add(&sp->srcu_work.entry, &srcu_boot_list);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(call_srcu);
> @@ -224,8 +224,8 @@ void __init srcu_init(void)
>  	srcu_init_done = true;
>  	while (!list_empty(&srcu_boot_list)) {
>  		sp = list_first_entry(&srcu_boot_list,
> -				      struct srcu_struct, srcu_boot_entry);
> -		list_del_init(&sp->srcu_boot_entry);
> +				      struct srcu_struct, srcu_work.entry);
> +		list_del_init(&sp->srcu_work.entry);
>  		schedule_work(&sp->srcu_work);
>  	}
>  }
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index e79c1929328f..36a2857c84e0 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -186,7 +186,6 @@ static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
>  	mutex_init(&sp->srcu_barrier_mutex);
>  	atomic_set(&sp->srcu_barrier_cpu_cnt, 0);
>  	INIT_DELAYED_WORK(&sp->work, process_srcu);
> -	INIT_LIST_HEAD(&sp->srcu_boot_entry);
>  	if (!is_static)
>  		sp->sda = alloc_percpu(struct srcu_data);
>  	init_srcu_struct_nodes(sp, is_static);
> @@ -709,8 +708,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
>  		if (likely(srcu_init_done))
>  			queue_delayed_work(rcu_gp_wq, &sp->work,
>  					   srcu_get_delay(sp));
> -		else if (list_empty(&sp->srcu_boot_entry))
> -			list_add(&sp->srcu_boot_entry, &srcu_boot_list);
> +		else if (list_empty(&sp->work.work.entry))
> +			list_add(&sp->work.work.entry, &srcu_boot_list);
>  	}
>  	spin_unlock_irqrestore_rcu_node(sp, flags);
>  }
> @@ -1324,10 +1323,10 @@ void __init srcu_init(void)
>  
>  	srcu_init_done = true;
>  	while (!list_empty(&srcu_boot_list)) {
> -		sp = list_first_entry(&srcu_boot_list,
> -				      struct srcu_struct, srcu_boot_entry);
> +		sp = list_first_entry(&srcu_boot_list, struct srcu_struct,
> +				      work.work.entry);
>  		check_init_srcu_struct(sp);
> -		list_del_init(&sp->srcu_boot_entry);
> +		list_del_init(&sp->work.work.entry);
>  		queue_work(rcu_gp_wq, &sp->work.work);
>  	}
>  }

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