Allow a CPU's rdp to quit the callback offlined mode. The switch happens on the target with IRQs disabled and rdp->nocb_lock held to avoid races between local callbacks handling and kthread offloaded callbacks handling.
nocb_cb kthread is first parked to avoid any future race with concurrent rcu_do_batch() executions. Then the cblist is set to offloaded so that the nocb_gp kthread ignores this rdp. Inspired-by: Paul E. McKenney <paul...@kernel.org> Signed-off-by: Frederic Weisbecker <frede...@kernel.org> Cc: Paul E. McKenney <paul...@kernel.org> Cc: Josh Triplett <j...@joshtriplett.org> Cc: Steven Rostedt <rost...@goodmis.org> Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> Cc: Lai Jiangshan <jiangshan...@gmail.com> Cc: Joel Fernandes <j...@joelfernandes.org> --- include/linux/rcupdate.h | 2 ++ kernel/rcu/rcu_segcblist.c | 4 +-- kernel/rcu/rcu_segcblist.h | 2 +- kernel/rcu/tree_plugin.h | 50 +++++++++++++++++++++++++++++++++++++- 4 files changed, 54 insertions(+), 4 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 2678a37c3169..1d3a4c37c3c1 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -96,8 +96,10 @@ static inline void rcu_user_exit(void) { } #ifdef CONFIG_RCU_NOCB_CPU void rcu_init_nohz(void); +void rcu_nocb_cpu_deoffload(int cpu); #else /* #ifdef CONFIG_RCU_NOCB_CPU */ static inline void rcu_init_nohz(void) { } +static inline void rcu_nocb_cpu_deoffload(int cpu) { } #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */ /** diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c index d8ea2bef5574..4bed48da7702 100644 --- a/kernel/rcu/rcu_segcblist.c +++ b/kernel/rcu/rcu_segcblist.c @@ -171,9 +171,9 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp) * Mark the specified rcu_segcblist structure as offloaded. This * structure must be empty. */ -void rcu_segcblist_offload(struct rcu_segcblist *rsclp) +void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload) { - rsclp->offloaded = 1; + rsclp->offloaded = offload; } #endif diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h index 4c1503a82492..8f7c6c34cb1b 100644 --- a/kernel/rcu/rcu_segcblist.h +++ b/kernel/rcu/rcu_segcblist.h @@ -83,7 +83,7 @@ void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp); void rcu_segcblist_init(struct rcu_segcblist *rsclp); void rcu_segcblist_disable(struct rcu_segcblist *rsclp); #ifdef CONFIG_RCU_NOCB_CPU -void rcu_segcblist_offload(struct rcu_segcblist *rsclp); +void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload); #endif bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp); bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp); diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index f19e81e0c691..c74a4df8d5f2 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -1943,6 +1943,10 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) { trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check")); raw_spin_lock_irqsave(&rdp->nocb_lock, flags); + if (!rcu_segcblist_is_offloaded(&rdp->cblist)) { + raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags); + continue; + } bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass); if (bypass_ncbs && (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) || @@ -2176,6 +2180,50 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp) do_nocb_deferred_wakeup_common(rdp); } +static void __rcu_nocb_rdp_deoffload(struct rcu_data *rdp) +{ + unsigned long flags; + struct rcu_node *rnp = rdp->mynode; + + printk("De-offloading %d\n", rdp->cpu); + kthread_park(rdp->nocb_cb_kthread); + + raw_spin_lock_irqsave(&rdp->nocb_lock, flags); + rcu_nocb_flush_bypass(rdp, NULL, jiffies); + raw_spin_lock_rcu_node(rnp); + rcu_segcblist_offload(&rdp->cblist, false); + raw_spin_unlock_rcu_node(rnp); + raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags); +} + +static long rcu_nocb_rdp_deoffload(void *arg) +{ + struct rcu_data *rdp = arg; + + WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id()); + __rcu_nocb_rdp_deoffload(rdp); + + return 0; +} + +void rcu_nocb_cpu_deoffload(int cpu) +{ + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); + + mutex_lock(&rcu_state.barrier_mutex); + cpus_read_lock(); + if (rcu_segcblist_is_offloaded(&rdp->cblist)) { + if (cpu_online(cpu)) { + work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp); + } else { + __rcu_nocb_rdp_deoffload(rdp); + } + cpumask_clear_cpu(cpu, rcu_nocb_mask); + } + cpus_read_unlock(); + mutex_unlock(&rcu_state.barrier_mutex); +} + void __init rcu_init_nohz(void) { int cpu; @@ -2218,7 +2266,7 @@ 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); + rcu_segcblist_offload(&rdp->cblist, true); } rcu_organize_nocb_kthreads(); } -- 2.25.0