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rcu-tasks: Update comments

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This commit updates comments to reflect the changes in the series
of commits that eliminated the full task-list scan.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-06-06 21:30:38 -07:00
parent 56096ecd5b
commit eea3423b16
1 changed files with 33 additions and 38 deletions
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71
kernel/rcu/tasks.h
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@ -1138,11 +1138,10 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
// 3. Avoids expensive read-side instructions, having overhead similar // 3. Avoids expensive read-side instructions, having overhead similar
// to that of Preemptible RCU. // to that of Preemptible RCU.
// //
// There are of course downsides. The grace-period code can send IPIs to // There are of course downsides. For example, the grace-period code
// CPUs, even when those CPUs are in the idle loop or in nohz_full userspace. // can send IPIs to CPUs, even when those CPUs are in the idle loop or
// It is necessary to scan the full tasklist, much as for Tasks RCU. There // in nohz_full userspace. If needed, these downsides can be at least
// is a single callback queue guarded by a single lock, again, much as for // partially remedied.
// Tasks RCU. If needed, these downsides can be at least partially remedied.
// //
// Perhaps most important, this variant of RCU does not affect the vanilla // Perhaps most important, this variant of RCU does not affect the vanilla
// flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace // flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace
@ -1155,38 +1154,30 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
// invokes these functions in this order: // invokes these functions in this order:
// //
// rcu_tasks_trace_pregp_step(): // rcu_tasks_trace_pregp_step():
// Initialize the count of readers and block CPU-hotplug operations. // Disables CPU hotplug, adds all currently executing tasks to the
// rcu_tasks_trace_pertask(), invoked on every non-idle task: // holdout list, then checks the state of all tasks that blocked
// Initialize per-task state and attempt to identify an immediate // or were preempted within their current RCU Tasks Trace read-side
// quiescent state for that task, or, failing that, attempt to // critical section, adding them to the holdout list if appropriate.
// set that task's .need_qs flag so that task's next outermost // Finally, this function re-enables CPU hotplug.
// rcu_read_unlock_trace() will report the quiescent state (in which // The ->pertask_func() pointer is NULL, so there is no per-task processing.
// case the count of readers is incremented). If both attempts fail,
// the task is added to a "holdout" list. Note that IPIs are used
// to invoke trc_read_check_handler() in the context of running tasks
// in order to avoid ordering overhead on common-case shared-variable
// accessses.
// rcu_tasks_trace_postscan(): // rcu_tasks_trace_postscan():
// Initialize state and attempt to identify an immediate quiescent // Invokes synchronize_rcu() to wait for late-stage exiting tasks
// state as above (but only for idle tasks), unblock CPU-hotplug // to finish exiting.
// operations, and wait for an RCU grace period to avoid races with
// tasks that are in the process of exiting.
// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty: // check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
// Scans the holdout list, attempting to identify a quiescent state // Scans the holdout list, attempting to identify a quiescent state
// for each task on the list. If there is a quiescent state, the // for each task on the list. If there is a quiescent state, the
// corresponding task is removed from the holdout list. // corresponding task is removed from the holdout list. Once this
// list is empty, the grace period has completed.
// rcu_tasks_trace_postgp(): // rcu_tasks_trace_postgp():
// Wait for the count of readers do drop to zero, reporting any stalls. // Provides the needed full memory barrier and does debug checks.
// Also execute full memory barriers to maintain ordering with code
// executing after the grace period.
// //
// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks. // The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
// //
// Pre-grace-period update-side code is ordered before the grace // Pre-grace-period update-side code is ordered before the grace period
// period via the ->cbs_lock and barriers in rcu_tasks_kthread(). // via the ->cbs_lock and barriers in rcu_tasks_kthread(). Pre-grace-period
// Pre-grace-period read-side code is ordered before the grace period by // read-side code is ordered before the grace period by atomic operations
// atomic_dec_and_test() of the count of readers (for IPIed readers) and by // on .b.need_qs flag of each task involved in this process, or by scheduler
// scheduler context-switch ordering (for locked-down non-running readers). // context-switch ordering (for locked-down non-running readers).
// The lockdep state must be outside of #ifdef to be useful. // The lockdep state must be outside of #ifdef to be useful.
#ifdef CONFIG_DEBUG_LOCK_ALLOC #ifdef CONFIG_DEBUG_LOCK_ALLOC
@ -1245,7 +1236,10 @@ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
} }
EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs); EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);
/* If we are the last reader, wake up the grace-period kthread. */ /*
* If we are the last reader, signal the grace-period kthread.
* Also remove from the per-CPU list of blocked tasks.
*/
void rcu_read_unlock_trace_special(struct task_struct *t) void rcu_read_unlock_trace_special(struct task_struct *t)
{ {
unsigned long flags; unsigned long flags;
@ -1336,9 +1330,9 @@ static void trc_read_check_handler(void *t_in)
if (unlikely(nesting < 0)) if (unlikely(nesting < 0))
goto reset_ipi; goto reset_ipi;
// Get here if the task is in a read-side critical section. Set // Get here if the task is in a read-side critical section.
// its state so that it will awaken the grace-period kthread upon // Set its state so that it will update state for the grace-period
// exit from that critical section. // kthread upon exit from that critical section.
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED); rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);
reset_ipi: reset_ipi:
@ -1387,7 +1381,7 @@ static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
return 0; // In QS, so done. return 0; // In QS, so done.
} }
if (nesting < 0) if (nesting < 0)
return -EINVAL; // QS transitioning, try again later. return -EINVAL; // Reader transitioning, try again later.
// The task is in a read-side critical section, so set up its // The task is in a read-side critical section, so set up its
// state so that it will update state upon exit from that critical // state so that it will update state upon exit from that critical
@ -1492,11 +1486,12 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
for_each_possible_cpu(cpu) for_each_possible_cpu(cpu)
WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu)); WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
// Disable CPU hotplug across the CPU scan. // Disable CPU hotplug across the CPU scan for the benefit of
// This also waits for all readers in CPU-hotplug code paths. // any IPIs that might be needed. This also waits for all readers
// in CPU-hotplug code paths.
cpus_read_lock(); cpus_read_lock();
// These smp_call_function_single() calls are serialized to // These rcu_tasks_trace_pertask_prep() calls are serialized to
// allow safe access to the hop list. // allow safe access to the hop list.
for_each_online_cpu(cpu) { for_each_online_cpu(cpu) {
rcu_read_lock(); rcu_read_lock();
@ -1608,7 +1603,7 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
{ {
struct task_struct *g, *t; struct task_struct *g, *t;
// Disable CPU hotplug across the holdout list scan. // Disable CPU hotplug across the holdout list scan for IPIs.
cpus_read_lock(); cpus_read_lock();
list_for_each_entry_safe(t, g, hop, trc_holdout_list) { list_for_each_entry_safe(t, g, hop, trc_holdout_list) {