Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

[Peter Zijlstra]

On Wed, Jun 24, 2015 at 10:10:17AM -0700, Paul E. McKenney wrote:
> OK, I will give this a try.  Of course, the counter needs to be
> initialized to 1 rather than zero, and it needs to be atomically
> decremented after all stop_one_cpu_nowait() invocations, otherwise you
> can get an early wakeup due to the usual race conditions.

Clever that.

How about something like this, it replaced mutex and start/done ticket
thing with an MCS style lockless FIFO queue.

I further uses the gpnum/completed thing to short circuit things if
we've waited long enough.

---
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3253,23 +3253,28 @@ void cond_synchronize_rcu(unsigned long
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
 
+struct exp_stop_state {
+	wait_queue_head_t	*wq;
+	atomic_t		count;
+};
+
 static int synchronize_sched_expedited_cpu_stop(void *data)
 {
+	struct exp_stop_state *ess = data;
+
 	/*
 	 * There must be a full memory barrier on each affected CPU
 	 * between the time that try_stop_cpus() is called and the
 	 * time that it returns.
-	 *
-	 * In the current initial implementation of cpu_stop, the
-	 * above condition is already met when the control reaches
-	 * this point and the following smp_mb() is not strictly
-	 * necessary.  Do smp_mb() anyway for documentation and
-	 * robustness against future implementation changes.
 	 */
-	smp_mb(); /* See above comment block. */
+	if (atomic_dec_and_test(&ess->count))
+		wake_up(ess->wq);
+
 	return 0;
 }
 
+static DEFINE_PER_CPU(struct cpu_stop_work, exp_stop_work);
+
 /**
  * synchronize_sched_expedited - Brute-force RCU-sched grace period
  *
@@ -3304,138 +3309,84 @@ static int synchronize_sched_expedited_c
  */
 void synchronize_sched_expedited(void)
 {
-	cpumask_var_t cm;
-	bool cma = false;
-	int cpu;
-	long firstsnap, s, snap;
-	int trycount = 0;
+	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(stop_wait);
+	struct exp_stop_state ess = {
+		.wq = &stop_wait,
+		.count = ATOMIC_INIT(1),
+	};
 	struct rcu_state *rsp = &rcu_sched_state;
+	struct expedited_queue_task {
+		struct expedited_queue_task *next;
+		struct task_struct *task;
+		int done;
+	} *prev, *next, entry = {
+		.task = current,
+	};
+	long gpnum;
+	int cpu;
 
-	/*
-	 * If we are in danger of counter wrap, just do synchronize_sched().
-	 * By allowing sync_sched_expedited_started to advance no more than
-	 * ULONG_MAX/8 ahead of sync_sched_expedited_done, we are ensuring
-	 * that more than 3.5 billion CPUs would be required to force a
-	 * counter wrap on a 32-bit system.  Quite a few more CPUs would of
-	 * course be required on a 64-bit system.
-	 */
-	if (ULONG_CMP_GE((ulong)atomic_long_read(&rsp->expedited_start),
-			 (ulong)atomic_long_read(&rsp->expedited_done) +
-			 ULONG_MAX / 8)) {
-		wait_rcu_gp(call_rcu_sched);
-		atomic_long_inc(&rsp->expedited_wrap);
-		return;
-	}
-
-	/*
-	 * Take a ticket.  Note that atomic_inc_return() implies a
-	 * full memory barrier.
-	 */
-	snap = atomic_long_inc_return(&rsp->expedited_start);
-	firstsnap = snap;
 	if (!try_get_online_cpus()) {
 		/* CPU hotplug operation in flight, fall back to normal GP. */
 		wait_rcu_gp(call_rcu_sched);
-		atomic_long_inc(&rsp->expedited_normal);
 		return;
 	}
 	WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));
 
-	/* Offline CPUs, idle CPUs, and any CPU we run on are quiescent. */
-	cma = zalloc_cpumask_var(&cm, GFP_KERNEL);
-	if (cma) {
-		cpumask_copy(cm, cpu_online_mask);
-		cpumask_clear_cpu(raw_smp_processor_id(), cm);
-		for_each_cpu(cpu, cm) {
-			struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+	smp_mb();
+	gpnum = smp_load_acquire(&rsp->gpnum);
 
-			if (!(atomic_add_return(0, &rdtp->dynticks) & 0x1))
-				cpumask_clear_cpu(cpu, cm);
+	/* MCS style queue 'lock' */
+	prev = xchg(&rsp->expedited_queue, &entry);
+	if (prev) {
+		WRITE_ONCE(prev->next, &entry);
+		for (;;) {
+			set_current_state(TASK_UNINTERRUPTIBLE);
+			if (smp_load_acquire(&entry.done))
+				break;
+			schedule();
 		}
-		if (cpumask_weight(cm) == 0)
-			goto all_cpus_idle;
+		__set_current_state(TASK_RUNNING);
 	}
 
 	/*
-	 * Each pass through the following loop attempts to force a
-	 * context switch on each CPU.
+	 * Check to see if someone else did our work for us, while we were
+	 * waiting on the queue.
 	 */
-	while (try_stop_cpus(cma ? cm : cpu_online_mask,
-			     synchronize_sched_expedited_cpu_stop,
-			     NULL) == -EAGAIN) {
-		put_online_cpus();
-		atomic_long_inc(&rsp->expedited_tryfail);
-
-		/* Check to see if someone else did our work for us. */
-		s = atomic_long_read(&rsp->expedited_done);
-		if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
-			/* ensure test happens before caller kfree */
-			smp_mb__before_atomic(); /* ^^^ */
-			atomic_long_inc(&rsp->expedited_workdone1);
-			free_cpumask_var(cm);
-			return;
-		}
-
-		/* No joy, try again later.  Or just synchronize_sched(). */
-		if (trycount++ < 10) {
-			udelay(trycount * num_online_cpus());
-		} else {
-			wait_rcu_gp(call_rcu_sched);
-			atomic_long_inc(&rsp->expedited_normal);
-			free_cpumask_var(cm);
-			return;
-		}
-
-		/* Recheck to see if someone else did our work for us. */
-		s = atomic_long_read(&rsp->expedited_done);
-		if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
-			/* ensure test happens before caller kfree */
-			smp_mb__before_atomic(); /* ^^^ */
-			atomic_long_inc(&rsp->expedited_workdone2);
-			free_cpumask_var(cm);
-			return;
-		}
+	if (ULONG_CMP_LT(gpnum, smp_load_acquire(&rsp->completed)))
+		goto unlock;
 
-		/*
-		 * Refetching sync_sched_expedited_started allows later
-		 * callers to piggyback on our grace period.  We retry
-		 * after they started, so our grace period works for them,
-		 * and they started after our first try, so their grace
-		 * period works for us.
-		 */
-		if (!try_get_online_cpus()) {
-			/* CPU hotplug operation in flight, use normal GP. */
-			wait_rcu_gp(call_rcu_sched);
-			atomic_long_inc(&rsp->expedited_normal);
-			free_cpumask_var(cm);
-			return;
-		}
-		snap = atomic_long_read(&rsp->expedited_start);
-		smp_mb(); /* ensure read is before try_stop_cpus(). */
+	/* Stop each CPU that is online, non-idle, and not us. */
+	for_each_online_cpu(cpu) {
+		struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+
+		/* Skip our CPU, */
+		if (raw_smp_processor_id() == cpu)
+			continue;
+
+		/* and any idle CPUs. */
+		if (!(atomic_add_return(0, &rdtp->dynticks) & 0x1))
+			continue;
+
+		atomic_inc(&ess.count);
+		stop_one_cpu_nowait(cpu, synchronize_sched_expedited_cpu_stop,
+				&ess, &per_cpu(exp_stop_work, cpu));
 	}
-	atomic_long_inc(&rsp->expedited_stoppedcpus);
+	atomic_dec(&ess.count);
 
-all_cpus_idle:
-	free_cpumask_var(cm);
+	wait_event(stop_wait, !atomic_read(&ess.count));
 
-	/*
-	 * Everyone up to our most recent fetch is covered by our grace
-	 * period.  Update the counter, but only if our work is still
-	 * relevant -- which it won't be if someone who started later
-	 * than we did already did their update.
-	 */
-	do {
-		atomic_long_inc(&rsp->expedited_done_tries);
-		s = atomic_long_read(&rsp->expedited_done);
-		if (ULONG_CMP_GE((ulong)s, (ulong)snap)) {
-			/* ensure test happens before caller kfree */
-			smp_mb__before_atomic(); /* ^^^ */
-			atomic_long_inc(&rsp->expedited_done_lost);
-			break;
-		}
-	} while (atomic_long_cmpxchg(&rsp->expedited_done, s, snap) != s);
-	atomic_long_inc(&rsp->expedited_done_exit);
+unlock:
+	/* MCS style queue 'unlock' */
+	next = READ_ONCE(entry.next);
+	if (!next) {
+		if (cmpxchg(&rsp->expedited_queue, &entry, NULL) == &entry)
+			goto done;
+		while (!(next = READ_ONCE(entry.next)))
+			cpu_relax();
+	}
+	smp_store_release(&next->done, 1);
 
+done:
 	put_online_cpus();
 }
 EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -483,17 +483,7 @@ struct rcu_state {
 						/*  _rcu_barrier(). */
 	/* End of fields guarded by barrier_mutex. */
 
-	atomic_long_t expedited_start;		/* Starting ticket. */
-	atomic_long_t expedited_done;		/* Done ticket. */
-	atomic_long_t expedited_wrap;		/* # near-wrap incidents. */
-	atomic_long_t expedited_tryfail;	/* # acquisition failures. */
-	atomic_long_t expedited_workdone1;	/* # done by others #1. */
-	atomic_long_t expedited_workdone2;	/* # done by others #2. */
-	atomic_long_t expedited_normal;		/* # fallbacks to normal. */
-	atomic_long_t expedited_stoppedcpus;	/* # successful stop_cpus. */
-	atomic_long_t expedited_done_tries;	/* # tries to update _done. */
-	atomic_long_t expedited_done_lost;	/* # times beaten to _done. */
-	atomic_long_t expedited_done_exit;	/* # times exited _done loop. */
+	void *expedited_queue;
 
 	unsigned long jiffies_force_qs;		/* Time at which to invoke */
 						/*  force_quiescent_state(). */
--
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