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Message-ID: <0ec35213-6bd0-467c-86fd-59db1d8b6ad9@paulmck-laptop>
Date: Wed, 3 Jul 2024 16:52:09 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
Joel Fernandes <joel@...lfernandes.org>,
Neeraj Upadhyay <neeraj.upadhyay@....com>,
Uladzislau Rezki <urezki@...il.com>,
Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 11/11 v2] rcu/nocb: Simplify (de-)offloading state machine
On Thu, Jul 04, 2024 at 12:56:40AM +0200, Frederic Weisbecker wrote:
> Now that the (de-)offloading process can only apply to offline CPUs,
> there is no more concurrency between rcu_core and nocb kthreads. Also
> the mutation now happens on empty queues.
>
> Therefore the state machine can be reduced to a single bit called
> SEGCBLIST_OFFLOADED. Simplify the transition as follows:
>
> * Upon offloading: queue the rdp to be added to the rcuog list and
> wait for the rcuog kthread to set the SEGCBLIST_OFFLOADED bit. Unpark
> rcuo kthread.
>
> * Upon de-offloading: Park rcuo kthread. Queue the rdp to be removed
> from the rcuog list and wait for the rcuog kthread to clear the
> SEGCBLIST_OFFLOADED bit.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
I have reverted the earlier patch and queued this one, thank you both!
If testing goes well, I will put the stack onto the nocb branch in
preparation for pushing it into the upcoming merge window.
Thanx, Paul
> ---
> include/linux/rcu_segcblist.h | 4 +-
> kernel/rcu/rcu_segcblist.c | 11 ---
> kernel/rcu/rcu_segcblist.h | 2 +-
> kernel/rcu/tree_nocb.h | 131 ++++++++++++++++------------------
> 4 files changed, 62 insertions(+), 86 deletions(-)
>
> diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
> index 1ef1bb54853d..2fdc2208f1ca 100644
> --- a/include/linux/rcu_segcblist.h
> +++ b/include/linux/rcu_segcblist.h
> @@ -185,9 +185,7 @@ struct rcu_cblist {
> * ----------------------------------------------------------------------------
> */
> #define SEGCBLIST_ENABLED BIT(0)
> -#define SEGCBLIST_LOCKING BIT(1)
> -#define SEGCBLIST_KTHREAD_GP BIT(2)
> -#define SEGCBLIST_OFFLOADED BIT(3)
> +#define SEGCBLIST_OFFLOADED BIT(1)
>
> struct rcu_segcblist {
> struct rcu_head *head;
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 1693ea22ef1b..298a2c573f02 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -260,17 +260,6 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
> rcu_segcblist_clear_flags(rsclp, SEGCBLIST_ENABLED);
> }
>
> -/*
> - * Mark the specified rcu_segcblist structure as offloaded (or not)
> - */
> -void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload)
> -{
> - if (offload)
> - rcu_segcblist_set_flags(rsclp, SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED);
> - else
> - rcu_segcblist_clear_flags(rsclp, SEGCBLIST_OFFLOADED);
> -}
> -
> /*
> * Does the specified rcu_segcblist structure contain callbacks that
> * are ready to be invoked?
> diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> index 7a0962dfee86..259904075636 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -89,7 +89,7 @@ static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
> static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
> {
> if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> - rcu_segcblist_test_flags(rsclp, SEGCBLIST_LOCKING))
> + rcu_segcblist_test_flags(rsclp, SEGCBLIST_OFFLOADED))
> return true;
>
> return false;
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 24daf606de0c..c8fc2f37c9ed 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -604,37 +604,33 @@ static void call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *head,
> }
> }
>
> -static int nocb_gp_toggle_rdp(struct rcu_data *rdp)
> +static void nocb_gp_toggle_rdp(struct rcu_data *rdp_gp, struct rcu_data *rdp)
> {
> struct rcu_segcblist *cblist = &rdp->cblist;
> unsigned long flags;
> - int ret;
>
> - rcu_nocb_lock_irqsave(rdp, flags);
> - if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED) &&
> - !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
> + /*
> + * Locking orders future de-offloaded callbacks enqueue against previous
> + * handling of this rdp. Ie: Make sure rcuog is done with this rdp before
> + * deoffloaded callbacks can be enqueued.
> + */
> + raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> + if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
> /*
> * Offloading. Set our flag and notify the offload worker.
> * We will handle this rdp until it ever gets de-offloaded.
> */
> - rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_GP);
> - ret = 1;
> - } else if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED) &&
> - rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
> + list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
> + rcu_segcblist_set_flags(cblist, SEGCBLIST_OFFLOADED);
> + } else {
> /*
> * De-offloading. Clear our flag and notify the de-offload worker.
> * We will ignore this rdp until it ever gets re-offloaded.
> */
> - rcu_segcblist_clear_flags(cblist, SEGCBLIST_KTHREAD_GP);
> - ret = 0;
> - } else {
> - WARN_ON_ONCE(1);
> - ret = -1;
> + list_del(&rdp->nocb_entry_rdp);
> + rcu_segcblist_clear_flags(cblist, SEGCBLIST_OFFLOADED);
> }
> -
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> -
> - return ret;
> + raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> }
>
> static void nocb_gp_sleep(struct rcu_data *my_rdp, int cpu)
> @@ -841,14 +837,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> }
>
> if (rdp_toggling) {
> - int ret;
> -
> - ret = nocb_gp_toggle_rdp(rdp_toggling);
> - if (ret == 1)
> - list_add_tail(&rdp_toggling->nocb_entry_rdp, &my_rdp->nocb_head_rdp);
> - else if (ret == 0)
> - list_del(&rdp_toggling->nocb_entry_rdp);
> -
> + nocb_gp_toggle_rdp(my_rdp, rdp_toggling);
> swake_up_one(&rdp_toggling->nocb_state_wq);
> }
>
> @@ -1018,16 +1007,11 @@ void rcu_nocb_flush_deferred_wakeup(void)
> }
> EXPORT_SYMBOL_GPL(rcu_nocb_flush_deferred_wakeup);
>
> -static int rdp_offload_toggle(struct rcu_data *rdp,
> - bool offload, unsigned long flags)
> - __releases(rdp->nocb_lock)
> +static int rcu_nocb_queue_toggle_rdp(struct rcu_data *rdp)
> {
> - struct rcu_segcblist *cblist = &rdp->cblist;
> struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
> bool wake_gp = false;
> -
> - rcu_segcblist_offload(cblist, offload);
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + unsigned long flags;
>
> raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
> // Queue this rdp for add/del to/from the list to iterate on rcuog
> @@ -1041,9 +1025,25 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
> return wake_gp;
> }
>
> +static bool rcu_nocb_rdp_deoffload_wait_cond(struct rcu_data *rdp)
> +{
> + unsigned long flags;
> + bool ret;
> +
> + /*
> + * Locking makes sure rcuog is done handling this rdp before deoffloaded
> + * enqueue can happen. Also it keeps the SEGCBLIST_OFFLOADED flag stable
> + * while the ->nocb_lock is held.
> + */
> + raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> + ret = !rcu_segcblist_test_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
> + raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> +
> + return ret;
> +}
> +
> static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
> {
> - struct rcu_segcblist *cblist = &rdp->cblist;
> unsigned long flags;
> int wake_gp;
> struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
> @@ -1059,48 +1059,34 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
> rcu_nocb_lock_irqsave(rdp, flags);
> WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
> WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
> + rcu_nocb_unlock_irqrestore(rdp, flags);
>
> - wake_gp = rdp_offload_toggle(rdp, false, flags);
> + wake_gp = rcu_nocb_queue_toggle_rdp(rdp);
>
> mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
> if (rdp_gp->nocb_gp_kthread) {
> if (wake_gp)
> wake_up_process(rdp_gp->nocb_gp_kthread);
>
> - swait_event_exclusive(rdp->nocb_state_wq,
> - !rcu_segcblist_test_flags(cblist,
> - SEGCBLIST_KTHREAD_GP));
> if (rdp->nocb_cb_kthread)
> kthread_park(rdp->nocb_cb_kthread);
> +
> + swait_event_exclusive(rdp->nocb_state_wq,
> + rcu_nocb_rdp_deoffload_wait_cond(rdp));
> } else {
> /*
> * No kthread to clear the flags for us or remove the rdp from the nocb list
> * to iterate. Do it here instead. Locking doesn't look stricly necessary
> * but we stick to paranoia in this rare path.
> */
> - rcu_nocb_lock_irqsave(rdp, flags);
> - rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> + rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
> + raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
>
> list_del(&rdp->nocb_entry_rdp);
> }
> - mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
>
> - /*
> - * Lock one last time to acquire latest callback updates from kthreads
> - * so we can later handle callbacks locally without locking.
> - */
> - rcu_nocb_lock_irqsave(rdp, flags);
> - /*
> - * Theoretically we could clear SEGCBLIST_LOCKING after the nocb
> - * lock is released but how about being paranoid for once?
> - */
> - rcu_segcblist_clear_flags(cblist, SEGCBLIST_LOCKING);
> - /*
> - * Without SEGCBLIST_LOCKING, we can't use
> - * rcu_nocb_unlock_irqrestore() anymore.
> - */
> - raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> + mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
>
> return 0;
> }
> @@ -1129,10 +1115,20 @@ int rcu_nocb_cpu_deoffload(int cpu)
> }
> EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
>
> -static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
> +static bool rcu_nocb_rdp_offload_wait_cond(struct rcu_data *rdp)
> {
> - struct rcu_segcblist *cblist = &rdp->cblist;
> unsigned long flags;
> + bool ret;
> +
> + raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> + ret = rcu_segcblist_test_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
> + raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> +
> + return ret;
> +}
> +
> +static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
> +{
> int wake_gp;
> struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
>
> @@ -1152,20 +1148,14 @@ static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
> WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
> WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
>
> - /*
> - * Can't use rcu_nocb_lock_irqsave() before SEGCBLIST_LOCKING
> - * is set.
> - */
> - raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> -
> - wake_gp = rdp_offload_toggle(rdp, true, flags);
> + wake_gp = rcu_nocb_queue_toggle_rdp(rdp);
> if (wake_gp)
> wake_up_process(rdp_gp->nocb_gp_kthread);
>
> - kthread_unpark(rdp->nocb_cb_kthread);
> -
> swait_event_exclusive(rdp->nocb_state_wq,
> - rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
> + rcu_nocb_rdp_offload_wait_cond(rdp));
> +
> + kthread_unpark(rdp->nocb_cb_kthread);
>
> return 0;
> }
> @@ -1340,8 +1330,7 @@ void __init rcu_init_nohz(void)
> rdp = per_cpu_ptr(&rcu_data, cpu);
> if (rcu_segcblist_empty(&rdp->cblist))
> rcu_segcblist_init(&rdp->cblist);
> - rcu_segcblist_offload(&rdp->cblist, true);
> - rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
> + rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
> }
> rcu_organize_nocb_kthreads();
> }
> --
> 2.34.1
>
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