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Subject[tip:core/rcu] rcu: Speed up calling of RCU tasks callbacks
Commit-ID:  c03be752d39dc64dcfda0ac8ce87fb10b1ee5621
Gitweb: https://git.kernel.org/tip/c03be752d39dc64dcfda0ac8ce87fb10b1ee5621
Author: Steven Rostedt (VMware) <rostedt@goodmis.org>
AuthorDate: Thu, 24 May 2018 18:49:46 -0400
Committer: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CommitDate: Thu, 12 Jul 2018 15:39:21 -0700

rcu: Speed up calling of RCU tasks callbacks

Joel Fernandes found that the synchronize_rcu_tasks() was taking a
significant amount of time. He demonstrated it with the following test:

# cd /sys/kernel/tracing
# while [ 1 ]; do x=1; done &
# echo '__schedule_bug:traceon' > set_ftrace_filter
# time echo '!__schedule_bug:traceon' > set_ftrace_filter;

real 0m1.064s
user 0m0.000s
sys 0m0.004s

Where it takes a little over a second to perform the synchronize,
because there's a loop that waits 1 second at a time for tasks to get
through their quiescent points when there's a task that must be waited
for.

After discussion we came up with a simple way to wait for holdouts but
increase the time for each iteration of the loop but no more than a
full second.

With the new patch we have:

# time echo '!__schedule_bug:traceon' > set_ftrace_filter;

real 0m0.131s
user 0m0.000s
sys 0m0.004s

Which drops it down to 13% of what the original wait time was.

Link: http://lkml.kernel.org/r/20180523063815.198302-2-joel@joelfernandes.org
Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/update.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 5783bdf86e5a..4c7c49c106ee 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -668,6 +668,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
struct rcu_head *list;
struct rcu_head *next;
LIST_HEAD(rcu_tasks_holdouts);
+ int fract;

/* Run on housekeeping CPUs by default. Sysadm can move if desired. */
housekeeping_affine(current, HK_FLAG_RCU);
@@ -749,13 +750,25 @@ static int __noreturn rcu_tasks_kthread(void *arg)
* holdouts. When the list is empty, we are done.
*/
lastreport = jiffies;
- while (!list_empty(&rcu_tasks_holdouts)) {
+
+ /* Start off with HZ/10 wait and slowly back off to 1 HZ wait*/
+ fract = 10;
+
+ for (;;) {
bool firstreport;
bool needreport;
int rtst;
struct task_struct *t1;

- schedule_timeout_interruptible(HZ);
+ if (list_empty(&rcu_tasks_holdouts))
+ break;
+
+ /* Slowly back off waiting for holdouts */
+ schedule_timeout_interruptible(HZ/fract);
+
+ if (fract > 1)
+ fract--;
+
rtst = READ_ONCE(rcu_task_stall_timeout);
needreport = rtst > 0 &&
time_after(jiffies, lastreport + rtst);
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