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rcu/nocb: (De-)offload callbacks on offline CPUs only

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Currently callbacks can be (de-)offloaded only on online CPUs. This
involves an overly elaborated state machine in order to make sure that
callbacks are always handled during the process while ensuring
synchronization between rcu_core and NOCB kthreads.

The only potential user of NOCB (de-)offloading appears to be a
nohz_full toggling interface through cpusets. And the general agreement
is now to work toward toggling the nohz_full state on offline CPUs to
simplify the whole picture.

Therefore, convert the (de-)offloading to only support offline CPUs.
This involves the following changes:

* Call rcu_barrier() before deoffloading. An offline offloaded CPU may
  still carry callbacks in its queue ignored by
  rcutree_migrate_callbacks(). Those callbacks must all be flushed
  before switching to a regular queue because no more kthreads will
  handle those before the CPU ever gets re-onlined.

  This means that further calls to rcu_barrier() will find an empty
  queue until the CPU goes through rcutree_report_cpu_starting(). As a
  result it is guaranteed that further rcu_barrier() won't try to lock
  the nocb_lock for that target and thus won't risk an imbalance.

  Therefore barrier_mutex doesn't need to be locked anymore upon
  deoffloading.

* Assume the queue is empty before offloading, as
  rcutree_migrate_callbacks() took care of everything.

  This means that further calls to rcu_barrier() will find an empty
  queue until the CPU goes through rcutree_report_cpu_starting(). As a
  result it is guaranteed that further rcu_barrier() won't risk a
  nocb_lock imbalance.

  Therefore barrier_mutex doesn't need to be locked anymore upon
  offloading.

* No need to flush bypass anymore.

Further simplifications will follow in upcoming patches.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
This commit is contained in:
Frederic Weisbecker 2024-05-30 15:45:46 +02:00 committed by Neeraj Upadhyay
parent 7121dd915a
commit 4e26ce4231
1 changed files with 21 additions and 61 deletions
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82
kernel/rcu/tree_nocb.h
View File

@ -1049,43 +1049,26 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
return wake_gp;
}
static long rcu_nocb_rdp_deoffload(void *arg)
static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
{
struct rcu_data *rdp = arg;
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int wake_gp;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
/*
* rcu_nocb_rdp_deoffload() may be called directly if
* rcuog/o[p] spawn failed, because at this time the rdp->cpu
* is not online yet.
*/
WARN_ON_ONCE((rdp->cpu != raw_smp_processor_id()) && cpu_online(rdp->cpu));
/* CPU must be offline, unless it's early boot */
WARN_ON_ONCE(cpu_online(rdp->cpu) && rdp->cpu != raw_smp_processor_id());
pr_info("De-offloading %d\n", rdp->cpu);
/* Flush all callbacks from segcblist and bypass */
rcu_barrier();
rcu_nocb_lock_irqsave(rdp, flags);
/*
* Flush once and for all now. This suffices because we are
* running on the target CPU holding ->nocb_lock (thus having
* interrupts disabled), and because rdp_offload_toggle()
* invokes rcu_segcblist_offload(), which clears SEGCBLIST_OFFLOADED.
* Thus future calls to rcu_segcblist_completely_offloaded() will
* return false, which means that future calls to rcu_nocb_try_bypass()
* will refuse to put anything into the bypass.
*/
WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
/*
* Start with invoking rcu_core() early. This way if the current thread
* happens to preempt an ongoing call to rcu_core() in the middle,
* leaving some work dismissed because rcu_core() still thinks the rdp is
* completely offloaded, we are guaranteed a nearby future instance of
* rcu_core() to catch up.
*/
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
rcu_segcblist_set_flags(cblist, SEGCBLIST_RCU_CORE);
invoke_rcu_core();
wake_gp = rdp_offload_toggle(rdp, false, flags);
mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
@ -1128,10 +1111,6 @@ static long rcu_nocb_rdp_deoffload(void *arg)
*/
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
/* Sanity check */
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
return 0;
}
@ -1142,18 +1121,16 @@ int rcu_nocb_cpu_deoffload(int cpu)
cpus_read_lock();
mutex_lock(&rcu_state.nocb_mutex);
mutex_lock(&rcu_state.barrier_mutex);
if (rcu_rdp_is_offloaded(rdp)) {
if (cpu_online(cpu)) {
ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
if (!cpu_online(cpu)) {
ret = rcu_nocb_rdp_deoffload(rdp);
if (!ret)
cpumask_clear_cpu(cpu, rcu_nocb_mask);
} else {
pr_info("NOCB: Cannot CB-deoffload offline CPU %d\n", rdp->cpu);
pr_info("NOCB: Cannot CB-deoffload online CPU %d\n", rdp->cpu);
ret = -EINVAL;
}
}
mutex_unlock(&rcu_state.barrier_mutex);
mutex_unlock(&rcu_state.nocb_mutex);
cpus_read_unlock();
@ -1161,15 +1138,14 @@ int rcu_nocb_cpu_deoffload(int cpu)
}
EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
static long rcu_nocb_rdp_offload(void *arg)
static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
{
struct rcu_data *rdp = arg;
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int wake_gp;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
WARN_ON_ONCE(cpu_online(rdp->cpu));
/*
* For now we only support re-offload, ie: the rdp must have been
* offloaded on boot first.
@ -1182,28 +1158,15 @@ static long rcu_nocb_rdp_offload(void *arg)
pr_info("Offloading %d\n", rdp->cpu);
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
/*
* Can't use rcu_nocb_lock_irqsave() before SEGCBLIST_LOCKING
* is set.
*/
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
/*
* We didn't take the nocb lock while working on the
* rdp->cblist with SEGCBLIST_LOCKING cleared (pure softirq/rcuc mode).
* Every modifications that have been done previously on
* rdp->cblist must be visible remotely by the nocb kthreads
* upon wake up after reading the cblist flags.
*
* The layout against nocb_lock enforces that ordering:
*
* __rcu_nocb_rdp_offload() nocb_cb_wait()/nocb_gp_wait()
* ------------------------- ----------------------------
* WRITE callbacks rcu_nocb_lock()
* rcu_nocb_lock() READ flags
* WRITE flags READ callbacks
* rcu_nocb_unlock() rcu_nocb_unlock()
*/
wake_gp = rdp_offload_toggle(rdp, true, flags);
if (wake_gp)
wake_up_process(rdp_gp->nocb_gp_kthread);
@ -1214,8 +1177,7 @@ static long rcu_nocb_rdp_offload(void *arg)
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
/*
* All kthreads are ready to work, we can finally relieve rcu_core() and
* enable nocb bypass.
* All kthreads are ready to work, we can finally enable nocb bypass.
*/
rcu_nocb_lock_irqsave(rdp, flags);
rcu_segcblist_clear_flags(cblist, SEGCBLIST_RCU_CORE);
@ -1231,18 +1193,16 @@ int rcu_nocb_cpu_offload(int cpu)
cpus_read_lock();
mutex_lock(&rcu_state.nocb_mutex);
mutex_lock(&rcu_state.barrier_mutex);
if (!rcu_rdp_is_offloaded(rdp)) {
if (cpu_online(cpu)) {
ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
if (!cpu_online(cpu)) {
ret = rcu_nocb_rdp_offload(rdp);
if (!ret)
cpumask_set_cpu(cpu, rcu_nocb_mask);
} else {
pr_info("NOCB: Cannot CB-offload offline CPU %d\n", rdp->cpu);
pr_info("NOCB: Cannot CB-offload online CPU %d\n", rdp->cpu);
ret = -EINVAL;
}
}
mutex_unlock(&rcu_state.barrier_mutex);
mutex_unlock(&rcu_state.nocb_mutex);
cpus_read_unlock();