linux-kernel - Re: [PATCH] rcupdate: reduce sys's overhead when rcu

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Date:	Mon, 27 Oct 2008 15:34:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcupdate: reduce sys's overhead when rcu_barrier()s
	called simultaneous

On Mon, Oct 27, 2008 at 01:49:49PM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> > On Fri, Oct 24, 2008 at 01:38:13PM +0800, Lai Jiangshan wrote:
> >> rcu_barrier() queues rcu_head on all cpus, it will brings
> >> large overhead for a large system which has a lots cpu.
> >> this fix reduces sys's overhead when rcu_barrier()s called
> >> simultaneous.
> > 
> > Hello, Jiangshan,
> > 
> > If we were to have problems with many concurrent rcu_barrier()
> > calls stacking up, this patch looks like it would be a reasonable
> > was of addressing those problems.
> > 
> > But do we really have problems with this at the moment?
> > 
> > 							Thanx, Paul
> 
> Hi, Paul,
> 
> 	Thanks, we do not have problems with this at the moment.
> 
> 	I suddenly had an association of ideas to synchronize_srcu(),
> so I made use of the ideas in synchronize_srcu() and this patch was made.

Absolutely nothing wrong with that!  ;-)

We should make sure that this patch is kept somewhere so that if
problems arise, it can be brought to bear quickly and easily.  One
approach would be to keep it in a git tree, but that would likely
incur merge overhead.  Another approach would be to have an RCU
web page maintaining patches such as this one.

Thoughts?

							Thanx, Paul

> 							Thanx, Lai.
> 
> 
> > 
> >> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> >> ---
> >> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> >> index ad63af8..734850b 100644
> >> --- a/kernel/rcupdate.c
> >> +++ b/kernel/rcupdate.c
> >> @@ -53,6 +53,9 @@ enum rcu_barrier {
> >>
> >>  static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
> >>  static atomic_t rcu_barrier_cpu_count;
> >> +static unsigned long rcu_barrier_completed;
> >> +static unsigned long rcu_barrier_completed_bh;
> >> +static unsigned long rcu_barrier_completed_sched;
> >>  static DEFINE_MUTEX(rcu_barrier_mutex);
> >>  static struct completion rcu_barrier_completion;
> >>
> >> @@ -60,7 +63,7 @@ static struct completion rcu_barrier_completion;
> >>   * Awaken the corresponding synchronize_rcu() instance now that a
> >>   * grace period has elapsed.
> >>   */
> >> -void wakeme_after_rcu(struct rcu_head  *head)
> >> +void wakeme_after_rcu(struct rcu_head *head)
> >>  {
> >>  	struct rcu_synchronize *rcu;
> >>
> >> @@ -113,11 +116,20 @@ static void rcu_barrier_func(void *type)
> >>   * Orchestrate the specified type of RCU barrier, waiting for all
> >>   * RCU callbacks of the specified type to complete.
> >>   */
> >> -static void _rcu_barrier(enum rcu_barrier type)
> >> +static void _rcu_barrier(enum rcu_barrier type, unsigned long *completed)
> >>  {
> >> +	unsigned long batch = ACCESS_ONCE(*completed);
> >>  	BUG_ON(in_interrupt());
> >>  	/* Take cpucontrol mutex to protect against CPU hotplug */
> >>  	mutex_lock(&rcu_barrier_mutex);
> >> +
> >> +	BUG_ON(*completed & 1);
> >> +	if ((*completed - batch) >= 2) {
> >> +		mutex_unlock(&rcu_barrier_mutex);
> >> +		return;
> >> +	}
> >> +	(*completed)++;
> >> +
> >>  	init_completion(&rcu_barrier_completion);
> >>  	/*
> >>  	 * Initialize rcu_barrier_cpu_count to 1, then invoke
> >> @@ -133,6 +145,7 @@ static void _rcu_barrier(enum rcu_barrier type)
> >>  	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> >>  		complete(&rcu_barrier_completion);
> >>  	wait_for_completion(&rcu_barrier_completion);
> >> +	(*completed)++;
> >>  	mutex_unlock(&rcu_barrier_mutex);
> >>  }
> >>
> >> @@ -141,7 +154,7 @@ static void _rcu_barrier(enum rcu_barrier type)
> >>   */
> >>  void rcu_barrier(void)
> >>  {
> >> -	_rcu_barrier(RCU_BARRIER_STD);
> >> +	_rcu_barrier(RCU_BARRIER_STD, &rcu_barrier_completed);
> >>  }
> >>  EXPORT_SYMBOL_GPL(rcu_barrier);
> >>
> >> @@ -150,7 +163,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier);
> >>   */
> >>  void rcu_barrier_bh(void)
> >>  {
> >> -	_rcu_barrier(RCU_BARRIER_BH);
> >> +	_rcu_barrier(RCU_BARRIER_BH, &rcu_barrier_completed_bh);
> >>  }
> >>  EXPORT_SYMBOL_GPL(rcu_barrier_bh);
> >>
> >> @@ -159,7 +172,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier_bh);
> >>   */
> >>  void rcu_barrier_sched(void)
> >>  {
> >> -	_rcu_barrier(RCU_BARRIER_SCHED);
> >> +	_rcu_barrier(RCU_BARRIER_SCHED, &rcu_barrier_completed_sched);
> >>  }
> >>  EXPORT_SYMBOL_GPL(rcu_barrier_sched);
> >>
> >>
> >>
> > 
> > 
> > 
> 
> 
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