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Subject[PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks
Date
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

RCU tasks callbacks can take at least 1 second before the callbacks are
executed. This happens even if the hold-out tasks enter their quiescent states
quickly. I noticed this when I was testing trampoline callback execution.

To test the trampoline freeing, I wrote a simple script:
cd /sys/kernel/debug/tracing/
echo '__schedule_bug:traceon' > set_ftrace_filter;
echo '!__schedule_bug:traceon' > set_ftrace_filter;

In the background I had simple bash while loop:
while [ 1 ]; do x=1; done &

Total time of completion of above commands in seconds:

With this patch:
real 0m0.179s
user 0m0.000s
sys 0m0.054s

Without this patch:
real 0m1.098s
user 0m0.000s
sys 0m0.053s

That's a greater than 6X speed up in performance. In order to accomplish
this, I am waiting for HZ/10 time before entering the hold-out checking
loop. The loop still preserves its checking of held tasks every 1 second
as before, in case this first test doesn't succeed.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zilstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: byungchul.park@lge.com
Cc: kernel-team@android.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/update.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 5783bdf86e5a..a28698e44b08 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -743,6 +743,12 @@ static int __noreturn rcu_tasks_kthread(void *arg)
*/
synchronize_srcu(&tasks_rcu_exit_srcu);

+ /*
+ * Wait a little bit incase held tasks are released
+ * during their next timer ticks.
+ */
+ schedule_timeout_interruptible(HZ/10);
+
/*
* Each pass through the following loop scans the list
* of holdout tasks, removing any that are no longer
@@ -755,7 +761,6 @@ static int __noreturn rcu_tasks_kthread(void *arg)
int rtst;
struct task_struct *t1;

- schedule_timeout_interruptible(HZ);
rtst = READ_ONCE(rcu_task_stall_timeout);
needreport = rtst > 0 &&
time_after(jiffies, lastreport + rtst);
@@ -768,6 +773,11 @@ static int __noreturn rcu_tasks_kthread(void *arg)
check_holdout_task(t, needreport, &firstreport);
cond_resched();
}
+
+ if (list_empty(&rcu_tasks_holdouts))
+ break;
+
+ schedule_timeout_interruptible(HZ);
}

/*
--
2.17.0.441.gb46fe60e1d-goog
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