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SubjectRe: [PATCH RFC] rcu: Make rcu_barrier() less disruptive
* 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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