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SubjectRe: [PATCH] rcupdate: reduce sys's overhead when rcu_barrier()s called simultaneous
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

> Signed-off-by: Lai Jiangshan <laijs@cn.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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