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Date:   Sat, 2 Jun 2018 04:45:22 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Byungchul Park <byungchul.park@lge.com>
Cc:     jiangshanlai@gmail.com, josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org,
        kernel-team@lge.com, joel@joelfernandes.org
Subject: Re: [PATCH] rcu: Check the range of jiffies_till_{first,next}_fqs
 when setting them
Reply-To: paulmck@linux.vnet.ibm.com
References: <1527818589-13476-1-git-send-email-byungchul.park@lge.com>
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On Fri, Jun 01, 2018 at 11:03:09AM +0900, Byungchul Park wrote:
> Currently, the range of jiffies_till_{first,next}_fqs are checked and
> adjusted on and on in the loop of rcu_gp_kthread on runtime.
> 
> However, it's enough to check them only when setting them, not every
> time in the loop. So make them handled on a setting time via sysfs.
> 
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>

Applied for further review and testing, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 45 ++++++++++++++++++++++++++++++++-------------
>  1 file changed, 32 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 4e96761..eb54d7d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -518,8 +518,38 @@ void rcu_all_qs(void)
>  static ulong jiffies_till_next_fqs = ULONG_MAX;
>  static bool rcu_kick_kthreads;
> 
> -module_param(jiffies_till_first_fqs, ulong, 0644);
> -module_param(jiffies_till_next_fqs, ulong, 0644);
> +static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp)
> +{
> +	ulong j;
> +	int ret = kstrtoul(val, 0, &j);
> +
> +	if (!ret)
> +		WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : j);
> +	return ret;
> +}
> +
> +static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param *kp)
> +{
> +	ulong j;
> +	int ret = kstrtoul(val, 0, &j);
> +
> +	if (!ret)
> +		WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : (j ?: 1));
> +	return ret;
> +}
> +
> +static struct kernel_param_ops first_fqs_jiffies_ops = {
> +	.set = param_set_first_fqs_jiffies,
> +	.get = param_get_ulong,
> +};
> +
> +static struct kernel_param_ops next_fqs_jiffies_ops = {
> +	.set = param_set_next_fqs_jiffies,
> +	.get = param_get_ulong,
> +};
> +
> +module_param_cb(jiffies_till_first_fqs, &first_fqs_jiffies_ops, &jiffies_till_first_fqs, 0644);
> +module_param_cb(jiffies_till_next_fqs, &next_fqs_jiffies_ops, &jiffies_till_next_fqs, 0644);
>  module_param(rcu_kick_kthreads, bool, 0644);
> 
>  /*
> @@ -2129,10 +2159,6 @@ static int __noreturn rcu_gp_kthread(void *arg)
>  		/* Handle quiescent-state forcing. */
>  		first_gp_fqs = true;
>  		j = jiffies_till_first_fqs;
> -		if (j > HZ) {
> -			j = HZ;
> -			jiffies_till_first_fqs = HZ;
> -		}
>  		ret = 0;
>  		for (;;) {
>  			if (!ret) {
> @@ -2167,13 +2193,6 @@ static int __noreturn rcu_gp_kthread(void *arg)
>  				WRITE_ONCE(rsp->gp_activity, jiffies);
>  				ret = 0; /* Force full wait till next FQS. */
>  				j = jiffies_till_next_fqs;
> -				if (j > HZ) {
> -					j = HZ;
> -					jiffies_till_next_fqs = HZ;
> -				} else if (j < 1) {
> -					j = 1;
> -					jiffies_till_next_fqs = 1;
> -				}
>  			} else {
>  				/* Deal with stray signal. */
>  				cond_resched_tasks_rcu_qs();
> -- 
> 1.9.1
>