by Neeraj Upadhyay (AMD)

[permalink] [raw]

Subject: Re: [PATCH 1/8] rcu/nocb: Make IRQs disablement symetric

On Sat, Dec 9, 2023 at 3:35 AM Frederic Weisbecker <[email protected]> wrote:
>
> Currently IRQs are disabled on call_rcu() and then depending on the
> context:
>
> * If the CPU is in nocb mode:
>
> - If the callback is enqueued in the bypass list, IRQs are re-enabled
> implictly by rcu_nocb_try_bypass()
>
> - If the callback is enqueued in the normal list, IRQs are re-enabled
> implicitly by __call_rcu_nocb_wake()
>
> * If the CPU is NOT in nocb mode, IRQs are reenabled explicitly from call_rcu()
>
> This makes the code a bit hard to follow, especially as it interleaves
> with nocb locking.
>
> To make the IRQ flags coverage clearer and also in order to prepare for
> moving all the nocb enqueue code to its own function, always re-enable
> the IRQ flags explicitly from call_rcu().
>
> Signed-off-by: Frederic Weisbecker <[email protected]>
> ---
> kernel/rcu/tree.c | 9 ++++++---
> kernel/rcu/tree_nocb.h | 20 +++++++++-----------
> 2 files changed, 15 insertions(+), 14 deletions(-)
>

Nit: s/symetric/symmetric/

Reviewed-by: Neeraj Upadhyay (AMD) <[email protected]>

Thanks
Neeraj

> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 52c2fdbc6363..74159c6d3bdf 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2697,8 +2697,10 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
> }
>
> check_cb_ovld(rdp);
> - if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy))
> + if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy)) {
> + local_irq_restore(flags);
> return; // Enqueued onto ->nocb_bypass, so just leave.
> + }
> // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
> rcu_segcblist_enqueue(&rdp->cblist, head);
> if (__is_kvfree_rcu_offset((unsigned long)func))
> @@ -2716,8 +2718,8 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
> __call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */
> } else {
> __call_rcu_core(rdp, head, flags);
> - local_irq_restore(flags);
> }
> + local_irq_restore(flags);
> }
>
> #ifdef CONFIG_RCU_LAZY
> @@ -4615,8 +4617,9 @@ void rcutree_migrate_callbacks(int cpu)
> __call_rcu_nocb_wake(my_rdp, true, flags);
> } else {
> rcu_nocb_unlock(my_rdp); /* irqs remain disabled. */
> - raw_spin_unlock_irqrestore_rcu_node(my_rnp, flags);
> + raw_spin_unlock_rcu_node(my_rnp); /* irqs remain disabled. */
> }
> + local_irq_restore(flags);
> if (needwake)
> rcu_gp_kthread_wake();
> lockdep_assert_irqs_enabled();
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 5598212d1f27..3f70fd0a2db4 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -532,9 +532,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> // 2. Both of these conditions are met:
> // a. The bypass list previously had only lazy CBs, and:
> // b. The new CB is non-lazy.
> - if (ncbs && (!bypass_is_lazy || lazy)) {
> - local_irq_restore(flags);
> - } else {
> + if (!ncbs || (bypass_is_lazy && !lazy)) {
> // No-CBs GP kthread might be indefinitely asleep, if so, wake.
> rcu_nocb_lock(rdp); // Rare during call_rcu() flood.
> if (!rcu_segcblist_pend_cbs(&rdp->cblist)) {
> @@ -544,7 +542,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> } else {
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> TPS("FirstBQnoWake"));
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> }
> }
> return true; // Callback already enqueued.
> @@ -570,7 +568,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> // If we are being polled or there is no kthread, just leave.
> t = READ_ONCE(rdp->nocb_gp_kthread);
> if (rcu_nocb_poll || !t) {
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> TPS("WakeNotPoll"));
> return;
> @@ -583,17 +581,17 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> rdp->qlen_last_fqs_check = len;
> // Only lazy CBs in bypass list
> if (lazy_len && bypass_len == lazy_len) {
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
> TPS("WakeLazy"));
> } else if (!irqs_disabled_flags(flags)) {
> /* ... if queue was empty ... */
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> wake_nocb_gp(rdp, false);
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> TPS("WakeEmpty"));
> } else {
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE,
> TPS("WakeEmptyIsDeferred"));
> }
> @@ -611,15 +609,15 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> if ((rdp->nocb_cb_sleep ||
> !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
> !timer_pending(&rdp->nocb_timer)) {
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
> TPS("WakeOvfIsDeferred"));
> } else {
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
> }
> } else {
> - rcu_nocb_unlock_irqrestore(rdp, flags);
> + rcu_nocb_unlock(rdp);
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
> }
> }
> --
> 2.42.1
>
>