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Date:   Mon, 8 Jul 2019 05:50:13 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Byungchul Park <byungchul.park@lge.com>
Cc:     josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@lge.com
Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable
Reply-To: paulmck@linux.ibm.com
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On Mon, Jul 08, 2019 at 03:00:09PM +0900, Byungchul Park wrote:
> jiffies_till_sched_qs is useless if it's readonly as it is used to set
> jiffies_to_sched_qs with its value regardless of first/next fqs jiffies.
> And it should be applied immediately on change through sysfs.

Actually, the intent was to only allow this to be changed at boot time.
Of course, if there is now a good reason to adjust it, it needs
to be adjustable.  So what situation is making you want to change
jiffies_till_sched_qs at runtime?  To what values is it proving useful
to adjust it?  What (if any) relationships between this timeout and the
various other RCU timeouts need to be maintained?  What changes to
rcutorture should be applied in order to test the ability to change
this at runtime?

							Thanx, Paul

> The function for setting jiffies_to_sched_qs,
> adjust_jiffies_till_sched_qs() will be called only if
> the value from sysfs != ULONG_MAX. And the value won't be adjusted
> unlike first/next fqs jiffies.
> 
> While at it, changed the positions of two module_param()s downward.
> 
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
>  kernel/rcu/tree.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a2f8ba2..a28e2fe 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -422,9 +422,7 @@ static int rcu_is_cpu_rrupt_from_idle(void)
>   * quiescent-state help from rcu_note_context_switch().
>   */
>  static ulong jiffies_till_sched_qs = ULONG_MAX;
> -module_param(jiffies_till_sched_qs, ulong, 0444);
>  static ulong jiffies_to_sched_qs; /* See adjust_jiffies_till_sched_qs(). */
> -module_param(jiffies_to_sched_qs, ulong, 0444); /* Display only! */
>  
>  /*
>   * Make sure that we give the grace-period kthread time to detect any
> @@ -450,6 +448,18 @@ static void adjust_jiffies_till_sched_qs(void)
>  	WRITE_ONCE(jiffies_to_sched_qs, j);
>  }
>  
> +static int param_set_sched_qs_jiffies(const char *val, const struct kernel_param *kp)
> +{
> +	ulong j;
> +	int ret = kstrtoul(val, 0, &j);
> +
> +	if (!ret && j != ULONG_MAX) {
> +		WRITE_ONCE(*(ulong *)kp->arg, j);
> +		adjust_jiffies_till_sched_qs();
> +	}
> +	return ret;
> +}
> +
>  static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp)
>  {
>  	ulong j;
> @@ -474,6 +484,11 @@ static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param
>  	return ret;
>  }
>  
> +static struct kernel_param_ops sched_qs_jiffies_ops = {
> +	.set = param_set_sched_qs_jiffies,
> +	.get = param_get_ulong,
> +};
> +
>  static struct kernel_param_ops first_fqs_jiffies_ops = {
>  	.set = param_set_first_fqs_jiffies,
>  	.get = param_get_ulong,
> @@ -484,8 +499,11 @@ static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param
>  	.get = param_get_ulong,
>  };
>  
> +module_param_cb(jiffies_till_sched_qs, &sched_qs_jiffies_ops, &jiffies_till_sched_qs, 0644);
>  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(jiffies_to_sched_qs, ulong, 0444); /* Display only! */
>  module_param(rcu_kick_kthreads, bool, 0644);
>  
>  static void force_qs_rnp(int (*f)(struct rcu_data *rdp));
> -- 
> 1.9.1
>