linux-kernel - Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

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Message-ID: <20160714192018.GM30154@twins.programming.kicks-ass.net>
Date:	Thu, 14 Jul 2016 21:20:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, tj@...nel.org,
	paulmck@...ux.vnet.ibm.com, john.stultz@...aro.org,
	dimitrysh@...gle.com, romlem@...gle.com, ccross@...gle.com,
	tkjos@...gle.com
Subject: Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

On Thu, Jul 14, 2016 at 08:43:51PM +0200, Oleg Nesterov wrote:
> 
> Oh well... I personally do not think this is what we want... Can't
> we just add the stupid rcu_sync_enter() into cgroup_init() at least
> for now? Yes, this means the unnecessary .sync() at boot time, but
> it will go away after cleanups I am going to send.

Something like so then?

---
--- a/include/linux/rcu_sync.h
+++ b/include/linux/rcu_sync.h
@@ -59,6 +59,7 @@ static inline bool rcu_sync_is_idle(stru
 }
 
 extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
+extern void rcu_sync_sabotage(struct rcu_sync *);
 extern void rcu_sync_enter(struct rcu_sync *);
 extern void rcu_sync_exit(struct rcu_sync *);
 extern void rcu_sync_dtor(struct rcu_sync *);
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5609,6 +5609,8 @@ int __init cgroup_init(void)
 	BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
 	BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));
 
+	rcu_sync_sabotage(&cgroup_threadgroup_rwsem.rss);
+
 	get_user_ns(init_cgroup_ns.user_ns);
 
 	mutex_lock(&cgroup_mutex);
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -83,6 +83,21 @@ void rcu_sync_init(struct rcu_sync *rsp,
 }
 
 /**
+ * rcu_sync_sabotage() - Sabotage a fresh rcu_sync instance
+ * @rsp: Pointer to rcu_sync structure to be sabotaged
+ *
+ * Must be called after rcu_sync_init() and before first use.
+ *
+ * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}() pairs
+ * turn into NO-OPs.
+ */
+void rcu_sync_sabotage(struct rcu_sync *rsp)
+{
+	rsp->gp_count++;
+	rsp->gp_state = !GP_IDLE;
+}
+
+/**
  * rcu_sync_enter() - Force readers onto slowpath
  * @rsp: Pointer to rcu_sync structure to use for synchronization
  *