[tip: core/rcu] rcu-tasks: Make the units of ->init_fract be jiffies

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     75dc2da5ecd65bdcbfc4d59b9d9b7342c61fe374
Gitweb:        https://git.kernel.org/tip/75dc2da5ecd65bdcbfc4d59b9d9b7342c61fe374
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Thu, 17 Sep 2020 16:17:17 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Fri, 06 Nov 2020 17:17:59 -08:00

rcu-tasks: Make the units of ->init_fract be jiffies

Currently, the units of ->init_fract are milliseconds while those of
->gp_sleep are jiffies.  For consistency with each other and with the
argument of schedule_timeout_idle(), this commit changes the units of
->init_fract to jiffies.

This change does affect the backoff algorithm, but only on systems where
HZ is not 1000, and even there the change makes more sense, given that the
current setup would "back off" to the same number of jiffies repeatedly.
In contrast, with this change, the number of jiffies waited increases
on each pass through the loop in the rcu_tasks_wait_gp() function.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tasks.h | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 0b45989..35bdcfd 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -335,8 +335,6 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 
 	// Start off with initial wait and slowly back off to 1 HZ wait.
 	fract = rtp->init_fract;
-	if (fract > HZ)
-		fract = HZ;
 
 	while (!list_empty(&holdouts)) {
 		bool firstreport;
@@ -345,10 +343,10 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 
 		/* Slowly back off waiting for holdouts */
 		set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
-		schedule_timeout_idle(HZ/fract);
+		schedule_timeout_idle(fract);
 
-		if (fract > 1)
-			fract--;
+		if (fract < HZ)
+			fract++;
 
 		rtst = READ_ONCE(rcu_task_stall_timeout);
 		needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
@@ -557,7 +555,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
 static int __init rcu_spawn_tasks_kthread(void)
 {
 	rcu_tasks.gp_sleep = HZ / 10;
-	rcu_tasks.init_fract = 10;
+	rcu_tasks.init_fract = HZ / 10;
 	rcu_tasks.pregp_func = rcu_tasks_pregp_step;
 	rcu_tasks.pertask_func = rcu_tasks_pertask;
 	rcu_tasks.postscan_func = rcu_tasks_postscan;
@@ -1178,12 +1176,12 @@ static int __init rcu_spawn_tasks_trace_kthread(void)
 {
 	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
 		rcu_tasks_trace.gp_sleep = HZ / 10;
-		rcu_tasks_trace.init_fract = 10;
+		rcu_tasks_trace.init_fract = HZ / 10;
 	} else {
 		rcu_tasks_trace.gp_sleep = HZ / 200;
 		if (rcu_tasks_trace.gp_sleep <= 0)
 			rcu_tasks_trace.gp_sleep = 1;
-		rcu_tasks_trace.init_fract = HZ / 5;
+		rcu_tasks_trace.init_fract = HZ / 200;
 		if (rcu_tasks_trace.init_fract <= 0)
 			rcu_tasks_trace.init_fract = 1;
 	}