[PATCH rcu 06/32] rcu-tasks: Remove rcu_tasks_trace_postgp() wait for counter

Paul E. McKenney
2022-06-20T22:55:12Z

Now that tasks are not removed from the list until they have responded to
any needed request for a quiescent state, it is no longer necessary to
wait for the trc_n_readers_need_end counter to go to zero. This commit
therefore removes that waiting code.

It is therefore also no longer necessary for rcu_tasks_trace_postgp() to
do the final decrement of this counter, so that code is also removed.
This in turn means that trc_n_readers_need_end counter itself can
be removed, as can the rcu_tasks_trace_iw irq_work structure and the
rcu_read_unlock_iw() function.

[ paulmck: Apply feedback from Zqiang. ]

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>
---
kernel/rcu/tasks.h | 62 +++-------------------------------------------
1 file changed, 3 insertions(+), 59 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 7bdc62606816b..561d24f7f73cc 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1192,9 +1192,6 @@ EXPORT_SYMBOL_GPL(rcu_trace_lock_map);

#ifdef CONFIG_TASKS_TRACE_RCU

-static atomic_t trc_n_readers_need_end; // Number of waited-for readers.
-static DECLARE_WAIT_QUEUE_HEAD(trc_wait); // List of holdout tasks.
-
// Record outstanding IPIs to each CPU. No point in sending two...
static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);

@@ -1241,16 +1238,6 @@ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
}
EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);

-/*
- * This irq_work handler allows rcu_read_unlock_trace() to be invoked
- * while the scheduler locks are held.
- */
-static void rcu_read_unlock_iw(struct irq_work *iwp)
-{
- wake_up(&trc_wait);
-}
-static DEFINE_IRQ_WORK(rcu_tasks_trace_iw, rcu_read_unlock_iw);
-
/* If we are the last reader, wake up the grace-period kthread. */
void rcu_read_unlock_trace_special(struct task_struct *t)
{
@@ -1267,8 +1254,6 @@ void rcu_read_unlock_trace_special(struct task_struct *t)
"%s: result = %d", __func__, result);
}
WRITE_ONCE(t->trc_reader_nesting, 0);
- if (nqs && atomic_dec_and_test(&trc_n_readers_need_end))
- irq_work_queue(&rcu_tasks_trace_iw);
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);

@@ -1313,8 +1298,7 @@ static void trc_read_check_handler(void *t_in)
// Get here if the task is in a read-side critical section. Set
// its state so that it will awaken the grace-period kthread upon
// exit from that critical section.
- if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED))
- atomic_inc(&trc_n_readers_need_end); // One more to wait on.
+ rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);

reset_ipi:
// Allow future IPIs to be sent on CPU and for task.
@@ -1367,10 +1351,8 @@ static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
// The task is in a read-side critical section, so set up its
// state so that it will awaken the grace-period kthread upon exit
// from that critical section.
- if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED)) {
- atomic_inc(&trc_n_readers_need_end); // One more to wait on.
+ if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED))
trc_add_holdout(t, bhp);
- }
return 0;
}

@@ -1436,9 +1418,6 @@ static void rcu_tasks_trace_pregp_step(void)
{
int cpu;

- // Allow for fast-acting IPIs.
- atomic_set(&trc_n_readers_need_end, 1);
-
// There shouldn't be any old IPIs, but...
for_each_possible_cpu(cpu)
WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
@@ -1581,10 +1560,6 @@ static void rcu_tasks_trace_empty_fn(void *unused)
static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
{
int cpu;
- bool firstreport;
- struct task_struct *g, *t;
- LIST_HEAD(holdouts);
- long ret;

// Wait for any lingering IPI handlers to complete. Note that
// if a CPU has gone offline or transitioned to userspace in the
@@ -1595,37 +1570,6 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
if (WARN_ON_ONCE(smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu))))
smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1);

- // Remove the safety count.
- smp_mb__before_atomic(); // Order vs. earlier atomics
- atomic_dec(&trc_n_readers_need_end);
- smp_mb__after_atomic(); // Order vs. later atomics
-
- // Wait for readers.
- set_tasks_gp_state(rtp, RTGS_WAIT_READERS);
- for (;;) {
- ret = wait_event_idle_exclusive_timeout(
- trc_wait,
- atomic_read(&trc_n_readers_need_end) == 0,
- READ_ONCE(rcu_task_stall_timeout));
- if (ret)
- break; // Count reached zero.
- // Stall warning time, so make a list of the offenders.
- rcu_read_lock();
- for_each_process_thread(g, t)
- if (rcu_ld_need_qs(t) & TRC_NEED_QS)
- trc_add_holdout(t, &holdouts);
- rcu_read_unlock();
- firstreport = true;
- list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list) {
- if (rcu_ld_need_qs(t) & TRC_NEED_QS)
- show_stalled_task_trace(t, &firstreport);
- trc_del_holdout(t); // Release task_struct reference.
- }
- if (firstreport)
- pr_err("INFO: rcu_tasks_trace detected stalls? (Counter/taskslist mismatch?)\n");
- show_stalled_ipi_trace();
- pr_err("\t%d holdouts\n", atomic_read(&trc_n_readers_need_end));
- }
smp_mb(); // Caller's code must be ordered after wakeup.
// Pairs with pretty much every ordering primitive.
}
@@ -1725,7 +1669,7 @@ void show_rcu_tasks_trace_gp_kthread(void)
{
char buf[64];

- sprintf(buf, "N%d h:%lu/%lu/%lu", atomic_read(&trc_n_readers_need_end),
+ sprintf(buf, "h:%lu/%lu/%lu",
data_race(n_heavy_reader_ofl_updates),
data_race(n_heavy_reader_updates),
data_race(n_heavy_reader_attempts));
--
2.31.1.189.g2e36527f23