Messages in this thread | | | Date | Thu, 15 Mar 2012 13:45:27 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH RFC] rcu: Make rcu_barrier() less disruptive |
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* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > The rcu_barrier() primitive interrupts each and every CPU, registering > a callback on every CPU. Once all of these callbacks have been invoked, > rcu_barrier() knows that every callback that was registered before > the call to rcu_barrier() has also been invoked. > > However, there is no point in registering a callback on a CPU that > currently has no callbacks, most especially if that CPU is in a > deep idle state. This commit therefore makes rcu_barrier() avoid > interrupting CPUs that have no callbacks. Doing this requires reworking > the handling of orphaned callbacks, otherwise callbacks could slip through > rcu_barrier()'s net by being orphaned from a CPU that rcu_barrier() had > not yet interrupted to a CPU that rcu_barrier() had already interrupted. > This reworking was needed anyway to take a first step towards weaning > RCU from the CPU_DYING notifier's use of stop_cpu().
Quoting Documentation/RCU/rcubarrier.txt:
"We instead need the rcu_barrier() primitive. This primitive is similar to synchronize_rcu(), but instead of waiting solely for a grace period to elapse, it also waits for all outstanding RCU callbacks to complete. Pseudo-code using rcu_barrier() is as follows:"
The patch you propose seems like a good approach to make rcu_barrier less disruptive, but everyone need to be aware that rcu_barrier() would quit having the side-effect of doing the equivalent of "synchronize_rcu()" from now on: within this new approach, in the case where there are no pending callbacks, rcu_barrier() could, AFAIU, return without waiting for the current grace period to complete.
Any use of rcu_barrier() that would assume that a synchronize_rcu() is implicit with the rcu_barrier() execution would be a bug anyway, but those might only show up after this patch is applied. I would therefore recommend to audit all rcu_barrier() users to ensure none is expecting rcu_barrier to act as a synchronize_rcu before pushing this change.
Thanks,
Mathieu
> > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index 403306b..8269656 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -75,6 +75,8 @@ static struct lock_class_key rcu_node_class[NUM_RCU_LVLS]; > .gpnum = -300, \ > .completed = -300, \ > .onofflock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.onofflock), \ > + .orphan_nxttail = &structname##_state.orphan_nxtlist, \ > + .orphan_donetail = &structname##_state.orphan_donelist, \ > .fqslock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.fqslock), \ > .n_force_qs = 0, \ > .n_force_qs_ngp = 0, \ > @@ -145,6 +147,13 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp); > unsigned long rcutorture_testseq; > unsigned long rcutorture_vernum; > > +/* State information for rcu_barrier() and friends. */ > + > +static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL}; > +static atomic_t rcu_barrier_cpu_count; > +static DEFINE_MUTEX(rcu_barrier_mutex); > +static DECLARE_WAIT_QUEUE_HEAD(rcu_barrier_wq); > + > /* > * Return true if an RCU grace period is in progress. The ACCESS_ONCE()s > * permit this function to be invoked without holding the root rcu_node > @@ -1311,7 +1320,60 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp) > #ifdef CONFIG_HOTPLUG_CPU > > /* > - * Move a dying CPU's RCU callbacks to online CPU's callback list. > + * Adopt the RCU callbacks from the specified rcu_state structure's > + * orphanage. The caller must hold the ->onofflock. > + */ > +static void rcu_adopt_orphan_cbs(struct rcu_state *rsp) > +{ > + int i; > + struct rcu_data *rdp = __this_cpu_ptr(rsp->rda); > + > + /* > + * If there is an rcu_barrier() operation in progress, then > + * only the task doing that operation is permitted to adopt > + * callbacks. To do otherwise breaks rcu_barrier() and friends > + * by causing them to fail to wait for the callbacks in the > + * orphanage. > + */ > + if (rsp->rcu_barrier_in_progress && > + rsp->rcu_barrier_in_progress != current) > + return; > + > + /* Do the accounting first. */ > + rdp->qlen_lazy += rsp->qlen_lazy; > + rdp->qlen += rsp->qlen; > + rdp->n_cbs_adopted += rsp->qlen; > + rsp->qlen_lazy = 0; > + rsp->qlen = 0; > + > + /* > + * We do not need a memory barrier here because the only way we > + * can get here if there is an rcu_barrier() in flight is if > + * we are the task doing the rcu_barrier(). > + */ > + > + /* First adopt the ready-to-invoke callbacks. */ > + if (rsp->orphan_donelist != NULL) { > + *rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL]; > + *rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist; > + for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--) > + if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL]) > + rdp->nxttail[i] = rsp->orphan_donetail; > + rsp->orphan_donelist = NULL; > + rsp->orphan_donetail = &rsp->orphan_donelist; > + } > + > + /* And then adopt the callbacks that still need a grace period. */ > + if (rsp->orphan_nxtlist != NULL) { > + *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist; > + rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail; > + rsp->orphan_nxtlist = NULL; > + rsp->orphan_nxttail = &rsp->orphan_nxtlist; > + } > +} > + > +/* > + * Move a dying CPU's RCU callbacks to the rcu_state structure's orphanage. > * Also record a quiescent state for this CPU for the current grace period. > * Synchronization and interrupt disabling are not required because > * this function executes in stop_machine() context. Therefore, cleanup > @@ -1325,64 +1387,67 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp) > static void rcu_cleanup_dying_cpu(struct rcu_state *rsp) > { > int i; > + unsigned long flags; > unsigned long mask; > - int receive_cpu = cpumask_any(cpu_online_mask); > + bool orphaned = 0; > struct rcu_data *rdp = this_cpu_ptr(rsp->rda); > - struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu); > RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */ > > - /* First, adjust the counts. */ > + /* Move the callbacks to the orphanage under ->onofflock protection. */ > + raw_spin_lock_irqsave(&rsp->onofflock, flags); > + > + /* First adjust the counts. */ > if (rdp->nxtlist != NULL) { > - receive_rdp->qlen_lazy += rdp->qlen_lazy; > - receive_rdp->qlen += rdp->qlen; > + rsp->qlen_lazy += rdp->qlen_lazy; > + rsp->qlen += rdp->qlen; > + rdp->n_cbs_orphaned += rdp->qlen; > rdp->qlen_lazy = 0; > rdp->qlen = 0; > + orphaned = 1; > } > > /* > - * Next, move ready-to-invoke callbacks to be invoked on some > - * other CPU. These will not be required to pass through another > - * grace period: They are done, regardless of CPU. > + * Next, move those callbacks still needing a grace period to > + * the orphanage, where some other CPU will pick them up. > + * Some of the callbacks might have gone partway through a grace > + * period, but that is too bad. They get to start over because we > + * cannot assume that grace periods are synchronized across CPUs. > + * We don't bother updating the ->nxttail[] array yet, instead > + * we just reset the whole thing later on. > */ > - if (rdp->nxtlist != NULL && > - rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) { > - struct rcu_head *oldhead; > - struct rcu_head **oldtail; > - struct rcu_head **newtail; > - > - oldhead = rdp->nxtlist; > - oldtail = receive_rdp->nxttail[RCU_DONE_TAIL]; > - rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL]; > - *rdp->nxttail[RCU_DONE_TAIL] = *oldtail; > - *receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead; > - newtail = rdp->nxttail[RCU_DONE_TAIL]; > - for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) { > - if (receive_rdp->nxttail[i] == oldtail) > - receive_rdp->nxttail[i] = newtail; > - if (rdp->nxttail[i] == newtail) > - rdp->nxttail[i] = &rdp->nxtlist; > - } > + if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) { > + *rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL]; > + rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL]; > + *rdp->nxttail[RCU_DONE_TAIL] = NULL; > } > > /* > - * Finally, put the rest of the callbacks at the end of the list. > - * The ones that made it partway through get to start over: We > - * cannot assume that grace periods are synchronized across CPUs. > - * (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but > - * this does not seem compelling. Not yet, anyway.) > + * Then move the ready-to-invoke callbacks to the orphanage, > + * where some other CPU will pick them up. These will not be > + * required to pass though another grace period: They are done. > */ > if (rdp->nxtlist != NULL) { > - *receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist; > - receive_rdp->nxttail[RCU_NEXT_TAIL] = > - rdp->nxttail[RCU_NEXT_TAIL]; > - receive_rdp->n_cbs_adopted += rdp->qlen; > - rdp->n_cbs_orphaned += rdp->qlen; > - > - rdp->nxtlist = NULL; > - for (i = 0; i < RCU_NEXT_SIZE; i++) > - rdp->nxttail[i] = &rdp->nxtlist; > + *rsp->orphan_donetail = rdp->nxtlist; > + rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL]; > } > > + /* Finally, initialize the rcu_data structure's list to empty. */ > + rdp->nxtlist = NULL; > + for (i = 0; i < RCU_NEXT_SIZE; i++) > + rdp->nxttail[i] = &rdp->nxtlist; > + > + /* > + * Wait up the rcu_barrier() task if there is one and if we > + * actually sent anything to the orphanage. Except that we > + * must delay the wakeup until ->onofflock is released to > + * avoid deadlock. > + */ > + if (!rsp->rcu_barrier_in_progress) > + orphaned = 0; > + raw_spin_unlock_irqrestore(&rsp->onofflock, flags); > + if (orphaned) > + wake_up(&rcu_barrier_wq); > + > /* > * Record a quiescent state for the dying CPU. This is safe > * only because we have already cleared out the callbacks. > @@ -1415,11 +1480,14 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp) > rcu_stop_cpu_kthread(cpu); > rcu_node_kthread_setaffinity(rnp, -1); > > - /* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */ > + /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */ > > /* Exclude any attempts to start a new grace period. */ > raw_spin_lock_irqsave(&rsp->onofflock, flags); > > + /* Collect the dead CPU's callbacks. */ > + rcu_adopt_orphan_cbs(rsp); > + > /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */ > mask = rdp->grpmask; /* rnp->grplo is constant. */ > do { > @@ -1456,6 +1524,10 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp) > > #else /* #ifdef CONFIG_HOTPLUG_CPU */ > > +static void rcu_adopt_orphan_cbs(struct rcu_state *rsp) > +{ > +} > + > static void rcu_cleanup_dying_cpu(struct rcu_state *rsp) > { > } > @@ -1524,9 +1596,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp) > rcu_is_callbacks_kthread()); > > /* Update count, and requeue any remaining callbacks. */ > - rdp->qlen_lazy -= count_lazy; > - rdp->qlen -= count; > - rdp->n_cbs_invoked += count; > if (list != NULL) { > *tail = rdp->nxtlist; > rdp->nxtlist = list; > @@ -1536,6 +1605,10 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp) > else > break; > } > + smp_mb(); /* List handling before counting for rcu_barrier(). */ > + rdp->qlen_lazy -= count_lazy; > + rdp->qlen -= count; > + rdp->n_cbs_invoked += count; > > /* Reinstate batch limit if we have worked down the excess. */ > if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark) > @@ -1824,13 +1897,14 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu), > rdp = this_cpu_ptr(rsp->rda); > > /* Add the callback to our list. */ > - *rdp->nxttail[RCU_NEXT_TAIL] = head; > - rdp->nxttail[RCU_NEXT_TAIL] = &head->next; > rdp->qlen++; > if (lazy) > rdp->qlen_lazy++; > else > rcu_idle_count_callbacks_posted(); > + smp_mb(); /* Count before adding callback for rcu_barrier(). */ > + *rdp->nxttail[RCU_NEXT_TAIL] = head; > + rdp->nxttail[RCU_NEXT_TAIL] = &head->next; > > if (__is_kfree_rcu_offset((unsigned long)func)) > trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func, > @@ -2169,15 +2243,10 @@ static int rcu_cpu_has_callbacks(int cpu) > rcu_preempt_cpu_has_callbacks(cpu); > } > > -static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL}; > -static atomic_t rcu_barrier_cpu_count; > -static DEFINE_MUTEX(rcu_barrier_mutex); > -static struct completion rcu_barrier_completion; > - > static void rcu_barrier_callback(struct rcu_head *notused) > { > if (atomic_dec_and_test(&rcu_barrier_cpu_count)) > - complete(&rcu_barrier_completion); > + wake_up(&rcu_barrier_wq); > } > > /* > @@ -2203,27 +2272,121 @@ static void _rcu_barrier(struct rcu_state *rsp, > void (*call_rcu_func)(struct rcu_head *head, > void (*func)(struct rcu_head *head))) > { > - BUG_ON(in_interrupt()); > + int cpu; > + unsigned long flags; > + struct rcu_data *rdp; > + struct rcu_head rh; > + > + init_rcu_head_on_stack(&rh); > + > /* Take mutex to serialize concurrent rcu_barrier() requests. */ > mutex_lock(&rcu_barrier_mutex); > - init_completion(&rcu_barrier_completion); > + > + smp_mb(); /* Prevent any prior operations from leaking in. */ > + > /* > - * Initialize rcu_barrier_cpu_count to 1, then invoke > - * rcu_barrier_func() on each CPU, so that each CPU also has > - * incremented rcu_barrier_cpu_count. Only then is it safe to > - * decrement rcu_barrier_cpu_count -- otherwise the first CPU > - * might complete its grace period before all of the other CPUs > - * did their increment, causing this function to return too > - * early. Note that on_each_cpu() disables irqs, which prevents > - * any CPUs from coming online or going offline until each online > - * CPU has queued its RCU-barrier callback. > + * Initialize the count to one rather than to zero in order to > + * avoid a too-soon return to zero in case of a short grace period > + * (or preemption of this task). Also flag this task as doing > + * an rcu_barrier(). This will prevent anyone else from adopting > + * orphaned callbacks, which could cause otherwise failure if a > + * CPU went offline and quickly came back online. To see this, > + * consider the following sequence of events: > + * > + * 1. We cause CPU 0 to post an rcu_barrier_callback() callback. > + * 2. CPU 1 goes offline, orphaning its callbacks. > + * 3. CPU 0 adopts CPU 1's orphaned callbacks. > + * 4. CPU 1 comes back online. > + * 5. We cause CPU 1 to post an rcu_barrier_callback() callback. > + * 6. Both rcu_barrier_callback() callbacks are invoked, awakening > + * us -- but before CPU 1's orphaned callbacks are invoked!!! > */ > atomic_set(&rcu_barrier_cpu_count, 1); > - on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1); > - if (atomic_dec_and_test(&rcu_barrier_cpu_count)) > - complete(&rcu_barrier_completion); > - wait_for_completion(&rcu_barrier_completion); > + raw_spin_lock_irqsave(&rsp->onofflock, flags); > + rsp->rcu_barrier_in_progress = current; > + raw_spin_unlock_irqrestore(&rsp->onofflock, flags); > + > + /* > + * Force every CPU with callbacks to register a new callback > + * that will tell us when all the preceding callbacks have > + * been invoked. If an offline CPU has callbacks, wait for > + * it to either come back online or to finish orphaning those > + * callbacks. > + */ > + for_each_possible_cpu(cpu) { > + preempt_disable(); > + rdp = per_cpu_ptr(rsp->rda, cpu); > + if (cpu_is_offline(cpu)) { > + preempt_enable(); > + while (cpu_is_offline(cpu) && > + ACCESS_ONCE(rdp->qlen)) > + schedule_timeout_interruptible(1); > + } else if (ACCESS_ONCE(rdp->qlen)) { > + smp_call_function_single(cpu, rcu_barrier_func, > + (void *)call_rcu_func, 1); > + preempt_enable(); > + } > + } > + > + /* > + * Force any ongoing CPU-hotplug operations to complete, > + * so that any callbacks from the outgoing CPUs are now in > + * the orphanage. > + */ > + cpu_maps_update_begin(); > + cpu_maps_update_done(); > + > + /* > + * Now that all online CPUs have rcu_barrier_callback() callbacks > + * posted, we can adopt all of the orphaned callbacks and place > + * an rcu_barrier_callback() callback after them. When that is done, > + * we are guaranteed to have an rcu_barrier_callback() callback > + * following every callback that could possibly have been > + * registered before _rcu_barrier() was called. > + */ > + raw_spin_lock_irqsave(&rsp->onofflock, flags); > + rcu_adopt_orphan_cbs(rsp); > + atomic_inc(&rcu_barrier_cpu_count); > + call_rcu_func(&rh, rcu_barrier_callback); > + raw_spin_unlock_irqrestore(&rsp->onofflock, flags); > + > + /* > + * Now that we have an rcu_barrier_callback() callback on each > + * CPU, and thus each counted, remove the initial count. > + */ > + atomic_dec(&rcu_barrier_cpu_count); > + smp_mb__after_atomic_dec(); > + > + /* > + * Loop waiting for all rcu_barrier_callback() callbacks to be > + * invoked. Adopt any orphaned callbacks in the meantime, just > + * in case one of the rcu_barrier_callback() callbacks is orphaned. > + */ > + while (atomic_read(&rcu_barrier_cpu_count) > 0) { > + wait_event(rcu_barrier_wq, > + atomic_read(&rcu_barrier_cpu_count) == 0 || > + ACCESS_ONCE(rsp->qlen)); > + if (ACCESS_ONCE(rsp->qlen)) { > + raw_spin_lock_irqsave(&rsp->onofflock, flags); > + rcu_adopt_orphan_cbs(rsp); > + raw_spin_unlock_irqrestore(&rsp->onofflock, flags); > + } > + } > + > + /* > + * Done, so let others adopt orphaned callbacks. But avoid > + * indefinite postponement of any additional orphans by adopting > + * one more time. > + */ > + raw_spin_lock_irqsave(&rsp->onofflock, flags); > + rcu_adopt_orphan_cbs(rsp); > + rsp->rcu_barrier_in_progress = NULL; > + raw_spin_unlock_irqrestore(&rsp->onofflock, flags); > + > + /* Other rcu_barrier() invocations can now safely proceed. */ > mutex_unlock(&rcu_barrier_mutex); > + > + destroy_rcu_head_on_stack(&rh); > } > > /** > diff --git a/kernel/rcutree.h b/kernel/rcutree.h > index 36ca28e..1e49c56 100644 > --- a/kernel/rcutree.h > +++ b/kernel/rcutree.h > @@ -371,6 +371,17 @@ struct rcu_state { > > raw_spinlock_t onofflock; /* exclude on/offline and */ > /* starting new GP. */ > + struct rcu_head *orphan_nxtlist; /* Orphaned callbacks that */ > + /* need a grace period. */ > + struct rcu_head **orphan_nxttail; /* Tail of above. */ > + struct rcu_head *orphan_donelist; /* Orphaned callbacks that */ > + /* are ready to invoke. */ > + struct rcu_head **orphan_donetail; /* Tail of above. */ > + long qlen_lazy; /* Number of lazy callbacks. */ > + long qlen; /* Total number of callbacks. */ > + struct task_struct *rcu_barrier_in_progress; > + /* Task doing rcu_barrier(), */ > + /* or NULL if no barrier. */ > raw_spinlock_t fqslock; /* Only one task forcing */ > /* quiescent states. */ > unsigned long jiffies_force_qs; /* Time at which to invoke */ > diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c > index ed459ed..d4bc16d 100644 > --- a/kernel/rcutree_trace.c > +++ b/kernel/rcutree_trace.c > @@ -271,13 +271,13 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp) > > gpnum = rsp->gpnum; > seq_printf(m, "c=%lu g=%lu s=%d jfq=%ld j=%x " > - "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu\n", > + "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n", > rsp->completed, gpnum, rsp->fqs_state, > (long)(rsp->jiffies_force_qs - jiffies), > (int)(jiffies & 0xffff), > rsp->n_force_qs, rsp->n_force_qs_ngp, > rsp->n_force_qs - rsp->n_force_qs_ngp, > - rsp->n_force_qs_lh); > + rsp->n_force_qs_lh, rsp->qlen_lazy, rsp->qlen); > for (rnp = &rsp->node[0]; rnp - &rsp->node[0] < NUM_RCU_NODES; rnp++) { > if (rnp->level != level) { > seq_puts(m, "\n"); >
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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