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Date: Wed, 15 Jul 2015 11:05:46 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Daniel Wagner <daniel.wagner@...-carit.de>,
Davidlohr Bueso <dave@...olabs.net>,
Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] rcu: Create rcu_sync infrastructure
On Sun, Jul 12, 2015 at 01:35:48AM +0200, Oleg Nesterov wrote:
> It is functionally equivalent to
>
> struct rcu_sync_struct {
> atomic_t counter;
> };
>
> static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
> {
If you add an smp_mb() here...
> return atomic_read(&rss->counter) == 0;
> }
>
> static inline void rcu_sync_enter(struct rcu_sync_struct *rss)
> {
> atomic_inc(&rss->counter);
> synchronize_sched();
> }
>
> static inline void rcu_sync_exit(struct rcu_sync_struct *rss)
> {
> synchronize_sched();
You should be able to demote the above synchronize_sched() to an
smp_mb__before_atomic(). Even rare writes should make this tradeoff
worthwhile.
I will try to call out the equivalent transformation below, for your
amusement if for no other reason. ;-)
> atomic_dec(&rss->counter);
> }
>
> except: it records the state and synchronize_sched() is only called by
> rcu_sync_enter() and only if necessary.
>
> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> include/linux/rcusync.h | 64 ++++++++++++++++++++++++++++
> kernel/rcu/Makefile | 2 +-
> kernel/rcu/sync.c | 108 +++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 173 insertions(+), 1 deletions(-)
> create mode 100644 include/linux/rcusync.h
> create mode 100644 kernel/rcu/sync.c
>
> diff --git a/include/linux/rcusync.h b/include/linux/rcusync.h
> new file mode 100644
> index 0000000..7858491
> --- /dev/null
> +++ b/include/linux/rcusync.h
> @@ -0,0 +1,64 @@
> +#ifndef _LINUX_RCUSYNC_H_
> +#define _LINUX_RCUSYNC_H_
> +
> +#include <linux/wait.h>
> +#include <linux/rcupdate.h>
> +
> +struct rcu_sync_struct {
> + int gp_state;
> + int gp_count;
> + wait_queue_head_t gp_wait;
> +
> + int cb_state;
> + struct rcu_head cb_head;
> +
> + void (*sync)(void);
> + void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
> +};
> +
> +#define ___RCU_SYNC_INIT(name) \
> + .gp_state = 0, \
> + .gp_count = 0, \
> + .gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait), \
> + .cb_state = 0
> +
> +#define __RCU_SCHED_SYNC_INIT(name) { \
> + ___RCU_SYNC_INIT(name), \
> + .sync = synchronize_sched, \
> + .call = call_rcu_sched, \
> +}
> +
> +#define __RCU_BH_SYNC_INIT(name) { \
> + ___RCU_SYNC_INIT(name), \
> + .sync = synchronize_rcu_bh, \
> + .call = call_rcu_bh, \
> +}
> +
> +#define __RCU_SYNC_INIT(name) { \
> + ___RCU_SYNC_INIT(name), \
> + .sync = synchronize_rcu, \
> + .call = call_rcu, \
> +}
> +
> +#define DEFINE_RCU_SCHED_SYNC(name) \
> + struct rcu_sync_struct name = __RCU_SCHED_SYNC_INIT(name)
> +
> +#define DEFINE_RCU_BH_SYNC(name) \
> + struct rcu_sync_struct name = __RCU_BH_SYNC_INIT(name)
> +
> +#define DEFINE_RCU_SYNC(name) \
> + struct rcu_sync_struct name = __RCU_SYNC_INIT(name)
> +
> +static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
> +{
smp_mb(); /* A: Ensure that reader sees last update. */
/* Pairs with B. */
> + return !rss->gp_state; /* GP_IDLE */
> +}
> +
> +enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
> +
> +extern void rcu_sync_init(struct rcu_sync_struct *, enum rcu_sync_type);
> +extern void rcu_sync_enter(struct rcu_sync_struct *);
> +extern void rcu_sync_exit(struct rcu_sync_struct *);
> +
> +#endif /* _LINUX_RCUSYNC_H_ */
> +
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 50a8084..61a1656 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -1,4 +1,4 @@
> -obj-y += update.o
> +obj-y += update.o sync.o
> obj-$(CONFIG_SRCU) += srcu.o
> obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
> obj-$(CONFIG_TREE_RCU) += tree.o
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> new file mode 100644
> index 0000000..f84176a
> --- /dev/null
> +++ b/kernel/rcu/sync.c
> @@ -0,0 +1,108 @@
> +
> +#include <linux/rcusync.h>
> +#include <linux/sched.h>
> +
> +enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
> +enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
> +
> +#define rss_lock gp_wait.lock
> +
> +void rcu_sync_init(struct rcu_sync_struct *rss, enum rcu_sync_type type)
> +{
> + memset(rss, 0, sizeof(*rss));
> + init_waitqueue_head(&rss->gp_wait);
> +
> + switch (type) {
> + case RCU_SYNC:
> + rss->sync = synchronize_rcu;
> + rss->call = call_rcu;
> + break;
> +
> + case RCU_SCHED_SYNC:
> + rss->sync = synchronize_sched;
> + rss->call = call_rcu_sched;
> + break;
> +
> + case RCU_BH_SYNC:
> + rss->sync = synchronize_rcu_bh;
> + rss->call = call_rcu_bh;
> + break;
> + }
> +}
> +
> +void rcu_sync_enter(struct rcu_sync_struct *rss)
> +{
> + bool need_wait, need_sync;
> +
> + spin_lock_irq(&rss->rss_lock);
> + need_wait = rss->gp_count++;
> + need_sync = rss->gp_state == GP_IDLE;
> + if (need_sync)
> + rss->gp_state = GP_PENDING;
> + spin_unlock_irq(&rss->rss_lock);
> +
> + BUG_ON(need_wait && need_sync);
> +
> + if (need_sync) {
> + rss->sync();
> + rss->gp_state = GP_PASSED;
> + wake_up_all(&rss->gp_wait);
> + } else if (need_wait) {
> + wait_event(rss->gp_wait, rss->gp_state == GP_PASSED);
> + } else {
> + /*
> + * Possible when there's a pending CB from a rcu_sync_exit().
> + * Nobody has yet been allowed the 'fast' path and thus we can
> + * avoid doing any sync(). The callback will get 'dropped'.
> + */
> + BUG_ON(rss->gp_state != GP_PASSED);
> + }
> +}
> +
> +static void rcu_sync_func(struct rcu_head *rcu)
> +{
> + struct rcu_sync_struct *rss =
> + container_of(rcu, struct rcu_sync_struct, cb_head);
> + unsigned long flags;
> +
> +
> + BUG_ON(rss->gp_state != GP_PASSED);
> + BUG_ON(rss->cb_state == CB_IDLE);
> +
> + spin_lock_irqsave(&rss->rss_lock, flags);
> + if (rss->gp_count) {
> + /*
> + * A new rcu_sync_begin() has happened; drop the callback.
> + */
> + rss->cb_state = CB_IDLE;
> + } else if (rss->cb_state == CB_REPLAY) {
> + /*
> + * A new rcu_sync_exit() has happened; requeue the callback
> + * to catch a later GP.
> + */
> + rss->cb_state = CB_PENDING;
> + rss->call(&rss->cb_head, rcu_sync_func);
> + } else {
> + /*
> + * We're at least a GP after rcu_sync_exit(); eveybody will now
> + * have observed the write side critical section. Let 'em rip!.
> + */
> + rss->cb_state = CB_IDLE;
> + rss->gp_state = GP_IDLE;
> + }
> + spin_unlock_irqrestore(&rss->rss_lock, flags);
> +}
> +
> +void rcu_sync_exit(struct rcu_sync_struct *rss)
> +{
> + spin_lock_irq(&rss->rss_lock);
smp_mb(); /* B: Make sure next readers see critical section. */
/* Pairs with A. */
> + if (!--rss->gp_count) {
At which point, I believe you can ditch the callback entirely, along
with ->cb_state.
So, what am I missing here? Are readers really so frequent that the
added read-side memory barrier is visible? Given that the current
code forces the readers to grab ->rss_lock during the trailing grace
period, any writer frequency at all will overwhelm the cost of the
read-side memory barrier.
Thanx, Paul
> + if (rss->cb_state == CB_IDLE) {
> + rss->cb_state = CB_PENDING;
> + rss->call(&rss->cb_head, rcu_sync_func);
> + } else if (rss->cb_state == CB_PENDING) {
> + rss->cb_state = CB_REPLAY;
> + }
> + }
> + spin_unlock_irq(&rss->rss_lock);
> +}
> --
> 1.5.5.1
>
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