On Mon, Sep 21, 2020 at 02:43:42PM +0200, Frederic Weisbecker wrote: > We'll need to be able to runtime offload and de-offload the processing > of callback for a given CPU. In order to support a smooth transition > from unlocked local processing (softirq/rcuc) to locked offloaded > processing (rcuop/rcuog) and the reverse, provide the necessary bits and > documentation for the state machine that will carry up all the steps to > enforce correctness while serving callbacks processing all along. > > 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/rcu_segcblist.h | 115 +++++++++++++++++++++++++++++++++- > kernel/rcu/rcu_segcblist.c | 1 + > kernel/rcu/rcu_segcblist.h | 12 +++- > kernel/rcu/tree.c | 3 + > 4 files changed, 128 insertions(+), 3 deletions(-) > > diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h > index dca2f39ee67f..67f09a912f96 100644 > --- a/include/linux/rcu_segcblist.h > +++ b/include/linux/rcu_segcblist.h > @@ -63,8 +63,121 @@ struct rcu_cblist { > #define RCU_NEXT_TAIL 3 > #define RCU_CBLIST_NSEGS 4 > > + > +/* > + * ==NOCB Offloading state machine== > + * > + * > + * > ---------------------------------------------------------------------------- > + * | SEGCBLIST_SOFTIRQ_ONLY > | > + * | > | > + * | Callbacks processed by rcu_core() from softirqs or local > | > + * | rcuc kthread, without holding nocb_lock. > | > + * > ---------------------------------------------------------------------------- > + * | > + * v > + * > ---------------------------------------------------------------------------- > + * | SEGCBLIST_OFFLOADED > | > + * | > | > + * | Callbacks processed by rcu_core() from softirqs or local > | > + * | rcuc kthread, while holding nocb_lock. Waking up CB and GP kthreads, > | > + * | allowing nocb_timer to be armed. > |
Whitespace nit before "allowing", just to show that I am paying attention. ;-) Don't we need to acquire rcu_state.barrier_mutex at this point? Otherwise rcu_barrier() could fail at this point. > + * > ---------------------------------------------------------------------------- > + * | > + * v > + * ----------------------------------- > + * | | > + * v v > + * --------------------------------------- > ----------------------------------| > + * | SEGCBLIST_OFFLOADED | | | SEGCBLIST_OFFLOADED | > | > + * | SEGCBLIST_KTHREAD_CB | | SEGCBLIST_KTHREAD_GP > | > + * | | | > | > + * | | | > | > + * | CB kthread woke up and | | GP kthread woke up and > | > + * | acknowledged SEGCBLIST_OFFLOADED. | | acknowledged > SEGCBLIST_OFFLOADED| > + * | Processes callbacks concurrently | | > | > + * | with rcu_core(), holding | | > | > + * | nocb_lock. | | > | > + * --------------------------------------- > ----------------------------------- > + * | | > + * ----------------------------------- > + * | > + * v > + * > |--------------------------------------------------------------------------| > + * | SEGCBLIST_OFFLOADED | > | > + * | SEGCBLIST_KTHREAD_CB | > | > + * | SEGCBLIST_KTHREAD_GP > | > + * | > | > + * | Kthreads handle callbacks holding nocb_lock, local rcu_core() stops > | > + * | handling callbacks. > | And rcu_state.barrier_mutex is dropped here. Or am I missing a trick here? I guess I will look at the later patches to get an initial estimate of an answer to this question. > + * > ---------------------------------------------------------------------------- > + */ > + > + > + > +/* > + * ==NOCB De-Offloading state machine== > + * > + * > + * > |--------------------------------------------------------------------------| > + * | SEGCBLIST_OFFLOADED | > | > + * | SEGCBLIST_KTHREAD_CB | > | > + * | SEGCBLIST_KTHREAD_GP > | > + * | > | > + * | CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core() > | > + * | ignores callbacks. > | And don't we also need to acquire rcu_state.barrier_mutex here? > + * > ---------------------------------------------------------------------------- > + * | > + * v > + * > |--------------------------------------------------------------------------| > + * | SEGCBLIST_KTHREAD_CB | > | > + * | SEGCBLIST_KTHREAD_GP > | > + * | > | > + * | CB/GP kthreads and local rcu_core() handle callbacks concurrently > | > + * | holding nocb_lock. Wake up CB and GP kthreads if necessary. > | > + * > ---------------------------------------------------------------------------- > + * | > + * v > + * ----------------------------------- > + * | | > + * v v > + * > ---------------------------------------------------------------------------| > + * | > | > + * | SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP > | > + * | | > | > + * | GP kthread woke up and | CB kthread woke up and > | > + * | acknowledged the fact that | acknowledged the fact that > | > + * | SEGCBLIST_OFFLOADED got cleared. | SEGCBLIST_OFFLOADED got > cleared. | > + * | | The CB kthread goes to sleep > | > + * | The callbacks from the target CPU | until it ever gets > re-offloaded. | > + * | will be ignored from the GP kthread | > | > + * | loop. | > | > + * > ---------------------------------------------------------------------------- > + * | | > + * ----------------------------------- > + * | > + * v > + * > ---------------------------------------------------------------------------- > + * | 0 > | > + * | > | > + * | Callbacks processed by rcu_core() from softirqs or local > | > + * | rcuc kthread, while holding nocb_lock. Forbid nocb_timer to be armed. > | > + * | Flush pending nocb_timer. Flush nocb bypass callbacks. > | And release rcu_state.barrier_mutex here? Thanx, Paul > + * > ---------------------------------------------------------------------------- > + * | > + * v > + * > ---------------------------------------------------------------------------- > + * | SEGCBLIST_SOFTIRQ_ONLY > | > + * | > | > + * | Callbacks processed by rcu_core() from softirqs or local > | > + * | rcuc kthread, without holding nocb_lock. > | > + * > ---------------------------------------------------------------------------- > + */ > #define SEGCBLIST_ENABLED BIT(0) > -#define SEGCBLIST_OFFLOADED BIT(1) > +#define SEGCBLIST_SOFTIRQ_ONLY BIT(1) > +#define SEGCBLIST_KTHREAD_CB BIT(2) > +#define SEGCBLIST_KTHREAD_GP BIT(3) > +#define SEGCBLIST_OFFLOADED BIT(4) > > struct rcu_segcblist { > struct rcu_head *head; > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c > index d131ef8940a0..31cc27ee98d8 100644 > --- a/kernel/rcu/rcu_segcblist.c > +++ b/kernel/rcu/rcu_segcblist.c > @@ -172,6 +172,7 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp) > */ > void rcu_segcblist_offload(struct rcu_segcblist *rsclp) > { > + rcu_segcblist_clear_flags(rsclp, SEGCBLIST_SOFTIRQ_ONLY); > rcu_segcblist_set_flags(rsclp, SEGCBLIST_OFFLOADED); > } > > diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h > index fc98761e3ee9..575896a2518b 100644 > --- a/kernel/rcu/rcu_segcblist.h > +++ b/kernel/rcu/rcu_segcblist.h > @@ -80,8 +80,16 @@ static inline bool rcu_segcblist_is_enabled(struct > rcu_segcblist *rsclp) > /* Is the specified rcu_segcblist offloaded? */ > static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp) > { > - return IS_ENABLED(CONFIG_RCU_NOCB_CPU) && > - rcu_segcblist_test_flags(rsclp, SEGCBLIST_OFFLOADED); > + if (IS_ENABLED(CONFIG_RCU_NOCB_CPU)) { > + /* > + * Complete de-offloading happens only when > SEGCBLIST_SOFTIRQ_ONLY > + * is set. > + */ > + if (!rcu_segcblist_test_flags(rsclp, SEGCBLIST_SOFTIRQ_ONLY)) > + return true; > + } > + > + return false; > } > > /* > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index c0286ce8fc03..b4292489db0c 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -96,6 +96,9 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, > rcu_data) = { > .dynticks_nesting = 1, > .dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE, > .dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR), > +#ifdef CONFIG_RCU_NOCB_CPU > + .cblist.flags = SEGCBLIST_SOFTIRQ_ONLY, > +#endif > }; > static struct rcu_state rcu_state = { > .level = { &rcu_state.node[0] }, > -- > 2.28.0 >