linux-kernel - [PATCH v3] rcu: Speed up calling of RCU tasks callbacks

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <20180524184057.10d32568@gandalf.local.home>
Date:   Thu, 24 May 2018 18:40:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com, Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [PATCH v3] rcu: Speed up calling of RCU tasks callbacks


From: Steven Rostedt (VMware) <rostedt@...dmis.org>

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.

I also noticed that there's a final HZ/10 wait at the end of the loop
for no apparent reason (at least there's nothing documenting why
there's a sleep), so I removed that as well.

With the new patch we have:

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

real	0m0.124s
user	0m0.000s
sys	0m0.004s

Which drops it down to 12% 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@...lfernandes.org>
Suggested-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 68fa19a5e7bd..0f6091097abc 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -715,6 +715,7 @@ 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);
@@ -796,13 +797,25 @@ 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);
@@ -848,7 +861,6 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 			list = next;
 			cond_resched();
 		}
-		schedule_timeout_uninterruptible(HZ/10);
 	}
 }