linux-kernel - Re: [PATCH] RCU: Remove have_rcu_nocb_mask from tree

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Date:   Sat, 18 Nov 2017 18:45:36 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Rakib Mullick <rakib.mullick@...il.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH]  RCU: Remove have_rcu_nocb_mask from tree_plugin.h

On Fri, Nov 17, 2017 at 09:40:15PM +0600, Rakib Mullick wrote:
> Currently have_rcu_nocb_mask is used to avoid double allocation of
> rcu_nocb_mask during boot up. Due to different representation of
> cpumask_var_t on different kernel config CPUMASK=y(or n) it was okay.
> But now we have a helper cpumask_available(), which can be utilized
> to check whether rcu_nocb_mask has been allocated or not without using
> a variable.
> 
> Removing the variable also reduces vmlinux size.
> 
> Unpatched version:
> text	   data	    bss	    dec	    hex	filename
> 13050393	7852470	14543408	35446271	21cddff	vmlinux
> 
> Patched version:
>  text	   data	    bss	    dec	    hex	filename
> 13050390	7852438	14543408	35446236	21cdddc	vmlinux
> 
> Signed-off-by: Rakib Mullick <rakib.mullick@...il.com>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>

Queued for review and testing, thank you!

							Thanx, Paul

> ---
> Patch applied on top of linus's tree (commit cf9b0772f2e41).
> 
>  kernel/rcu/tree_plugin.h | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index db85ca3..13a8e08 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -61,7 +61,6 @@ DEFINE_PER_CPU(char, rcu_cpu_has_work);
> 
>  #ifdef CONFIG_RCU_NOCB_CPU
>  static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
> -static bool have_rcu_nocb_mask;	    /* Was rcu_nocb_mask allocated? */
>  static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */
>  #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
> 
> @@ -1752,7 +1751,6 @@ static void increment_cpu_stall_ticks(void)
>  static int __init rcu_nocb_setup(char *str)
>  {
>  	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
> -	have_rcu_nocb_mask = true;
>  	cpulist_parse(str, rcu_nocb_mask);
>  	return 1;
>  }
> @@ -1801,7 +1799,7 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
>  /* Is the specified CPU a no-CBs CPU? */
>  bool rcu_is_nocb_cpu(int cpu)
>  {
> -	if (have_rcu_nocb_mask)
> +	if (cpumask_available(rcu_nocb_mask))
>  		return cpumask_test_cpu(cpu, rcu_nocb_mask);
>  	return false;
>  }
> @@ -2295,14 +2293,13 @@ void __init rcu_init_nohz(void)
>  		need_rcu_nocb_mask = true;
>  #endif /* #if defined(CONFIG_NO_HZ_FULL) */
> 
> -	if (!have_rcu_nocb_mask && need_rcu_nocb_mask) {
> +	if (!cpumask_available(rcu_nocb_mask) && need_rcu_nocb_mask) {
>  		if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
>  			pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
>  			return;
>  		}
> -		have_rcu_nocb_mask = true;
>  	}
> -	if (!have_rcu_nocb_mask)
> +	if (!cpumask_available(rcu_nocb_mask))
>  		return;
> 
>  #if defined(CONFIG_NO_HZ_FULL)
> @@ -2428,7 +2425,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
>  	struct rcu_data *rdp_leader = NULL;  /* Suppress misguided gcc warn. */
>  	struct rcu_data *rdp_prev = NULL;
> 
> -	if (!have_rcu_nocb_mask)
> +	if (!cpumask_available(rcu_nocb_mask))
>  		return;
>  	if (ls == -1) {
>  		ls = int_sqrt(nr_cpu_ids);
> -- 
> 2.9.3
>