[PATCH v4] fs/buffer.c: update per-CPU bh_lru cache via RCU

[Marcelo Tosatti <mtosatti@redhat.com>]

For certain types of applications (for example PLC software or
RAN processing), upon occurrence of an event, it is necessary to
complete a certain task in a maximum amount of time (deadline).

One way to express this requirement is with a pair of numbers, 
deadline time and execution time, where:

	* deadline time: length of time between event and deadline.
	* execution time: length of time it takes for processing of event
			  to occur on a particular hardware platform
			  (uninterrupted).

The particular values depend on use-case. For the case
where the realtime application executes in a virtualized
guest, an IPI which must be serviced in the host will cause 
the following sequence of events:

	1) VM-exit
	2) execution of IPI (and function call)
	3) VM-entry

Which causes an excess of 50us latency as observed by cyclictest
(this violates the latency requirement of vRAN application with 1ms TTI,
for example).

invalidate_bh_lrus calls an IPI on each CPU that has non empty
per-CPU cache:

	on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);

To avoid the IPI, free the per-CPU caches remotely via RCU.
Two bh_lrus structures for each CPU are allocated: one is being
used (assigned to per-CPU bh_lru pointer), and the other is
being freed (or idle).

An alternative solution would be to protect the fast path 
(__find_get_block) with a per-CPU spinlock. Then grab the 
lock from invalidate_bh_lru, when evaluating whether a given
CPUs buffer_head cache should be invalidated.
This solution would slow down the fast path.

Numbers (16 vCPU guest) for the following test:

for i in `seq 0 50`;
	mount -o loop alpine-standard-3.17.1-x86_64.iso /mnt/loop
	umount /mnt/loop
done

Where the time being measured is time between invalidate_bh_lrus 
function call start and return.

Unpatched: average is 2us
	     ┌                                        ┐
[ 0.0,  2.0) ┤████████████████████████▊ 53
[ 2.0,  4.0) ┤████████████████████████████████████  77
[ 4.0,  6.0) ┤████████▍ 18
[ 6.0,  8.0) ┤▌ 1
[ 8.0, 10.0) ┤  0
[10.0, 12.0) ┤  0
[12.0, 14.0) ┤▌ 1
[14.0, 16.0) ┤  0
[16.0, 18.0) ┤▌ 1
	     └                                        ┘
			   Frequency

Patched: average is 16us

	     ┌                                        ┐
[ 0.0, 10.0) ┤██████████████████▍ 35
[10.0, 20.0) ┤████████████████████████████████████  69
[20.0, 30.0) ┤██████████████████▍ 35
[30.0, 40.0) ┤████▎ 8
[40.0, 50.0) ┤█▌ 3
[50.0, 60.0) ┤█▏ 2
	     └                                        ┘
			   Frequency

The fact that invalidate_bh_lru() is now serialized should not be 
an issue, since invalidate_bdev does:

/* Invalidate clean unused buffers and pagecache. */
void invalidate_bdev(struct block_device *bdev)
{
	struct address_space *mapping = bdev->bd_inode->i_mapping;

	if (mapping->nrpages) {
		invalidate_bh_lrus();
		lru_add_drain_all();    /* make sure all lru add caches are flushed */
		invalidate_mapping_pages(mapping, 0, -1);
	}
}

Where lru_add_drain_all() is serialized by a single mutex lock
(and there have been no reported use cases where this
serialization is an issue).

Regarding scalability, considering the results above where 
it takes 16us to execute invalidate_bh_lrus on 16 CPUs
(where 8us are taken by synchronize_rcu_expedited),
we can assume 500ns per CPU. For a system with 
1024 CPUs, we can infer 8us + 1024*500ns ~= 500us
(which seems acceptable).

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---

v4: improved changelog, no code change	(Dave Chinner)
v3: fix CPU hotplug
v2: fix sparse warnings (kernel test robot)

diff --git a/fs/buffer.c b/fs/buffer.c
index 9e1e2add541e..e9b4d579eff0 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1246,7 +1246,21 @@ struct bh_lru {
 	struct buffer_head *bhs[BH_LRU_SIZE];
 };
 
-static DEFINE_PER_CPU(struct bh_lru, bh_lrus) = {{ NULL }};
+
+/*
+ * Allocate two bh_lrus structures for each CPU. bh_lru points to the
+ * one that is currently in use, and the update path does
+ * (consider cpu->bh_lru = bh_lrus[0]).
+ *
+ * cpu->bh_lrup = bh_lrus[1]
+ * synchronize_rcu()
+ * free bh's in bh_lrus[0]
+ */
+static unsigned int bh_lru_idx;
+static DEFINE_PER_CPU(struct bh_lru, bh_lrus[2]) = {{{ NULL }}, {{NULL}}};
+static DEFINE_PER_CPU(struct bh_lru __rcu *, bh_lrup);
+
+static DEFINE_MUTEX(bh_lru_invalidate_mutex);
 
 #ifdef CONFIG_SMP
 #define bh_lru_lock()	local_irq_disable()
@@ -1288,16 +1302,19 @@ static void bh_lru_install(struct buffer_head *bh)
 		return;
 	}
 
-	b = this_cpu_ptr(&bh_lrus);
+	rcu_read_lock();
+	b = rcu_dereference(per_cpu(bh_lrup, smp_processor_id()));
 	for (i = 0; i < BH_LRU_SIZE; i++) {
 		swap(evictee, b->bhs[i]);
 		if (evictee == bh) {
+			rcu_read_unlock();
 			bh_lru_unlock();
 			return;
 		}
 	}
 
 	get_bh(bh);
+	rcu_read_unlock();
 	bh_lru_unlock();
 	brelse(evictee);
 }
@@ -1309,28 +1326,32 @@ static struct buffer_head *
 lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
 {
 	struct buffer_head *ret = NULL;
+	struct bh_lru *lru;
 	unsigned int i;
 
 	check_irqs_on();
 	bh_lru_lock();
+	rcu_read_lock();
+
+	lru = rcu_dereference(per_cpu(bh_lrup, smp_processor_id()));
 	for (i = 0; i < BH_LRU_SIZE; i++) {
-		struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);
+		struct buffer_head *bh = lru->bhs[i];
 
 		if (bh && bh->b_blocknr == block && bh->b_bdev == bdev &&
 		    bh->b_size == size) {
 			if (i) {
 				while (i) {
-					__this_cpu_write(bh_lrus.bhs[i],
-						__this_cpu_read(bh_lrus.bhs[i - 1]));
+					lru->bhs[i] = lru->bhs[i - 1];
 					i--;
 				}
-				__this_cpu_write(bh_lrus.bhs[0], bh);
+				lru->bhs[0] = bh;
 			}
 			get_bh(bh);
 			ret = bh;
 			break;
 		}
 	}
+	rcu_read_unlock();
 	bh_lru_unlock();
 	return ret;
 }
@@ -1424,35 +1445,54 @@ static void __invalidate_bh_lrus(struct bh_lru *b)
 		b->bhs[i] = NULL;
 	}
 }
-/*
- * invalidate_bh_lrus() is called rarely - but not only at unmount.
- * This doesn't race because it runs in each cpu either in irq
- * or with preempt disabled.
- */
-static void invalidate_bh_lru(void *arg)
-{
-	struct bh_lru *b = &get_cpu_var(bh_lrus);
-
-	__invalidate_bh_lrus(b);
-	put_cpu_var(bh_lrus);
-}
 
 bool has_bh_in_lru(int cpu, void *dummy)
 {
-	struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
+	struct bh_lru *b;
 	int i;
-	
+
+	rcu_read_lock();
+	b = rcu_dereference(per_cpu(bh_lrup, cpu));
 	for (i = 0; i < BH_LRU_SIZE; i++) {
-		if (b->bhs[i])
+		if (b->bhs[i]) {
+			rcu_read_unlock();
 			return true;
+		}
 	}
 
+	rcu_read_unlock();
 	return false;
 }
 
+/*
+ * invalidate_bh_lrus() is called rarely - but not only at unmount.
+ */
 void invalidate_bh_lrus(void)
 {
-	on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);
+	int cpu, oidx;
+
+	mutex_lock(&bh_lru_invalidate_mutex);
+	cpus_read_lock();
+	oidx = bh_lru_idx;
+	bh_lru_idx++;
+	if (bh_lru_idx >= 2)
+		bh_lru_idx = 0;
+
+	/* Assign the per-CPU bh_lru pointer */
+	for_each_online_cpu(cpu)
+		rcu_assign_pointer(per_cpu(bh_lrup, cpu),
+				   per_cpu_ptr(&bh_lrus[bh_lru_idx], cpu));
+	synchronize_rcu_expedited();
+
+	for_each_online_cpu(cpu) {
+		struct bh_lru *b = per_cpu_ptr(&bh_lrus[oidx], cpu);
+
+		bh_lru_lock();
+		__invalidate_bh_lrus(b);
+		bh_lru_unlock();
+	}
+	cpus_read_unlock();
+	mutex_unlock(&bh_lru_invalidate_mutex);
 }
 EXPORT_SYMBOL_GPL(invalidate_bh_lrus);
 
@@ -1465,8 +1505,10 @@ void invalidate_bh_lrus_cpu(void)
 	struct bh_lru *b;
 
 	bh_lru_lock();
-	b = this_cpu_ptr(&bh_lrus);
+	rcu_read_lock();
+	b = rcu_dereference(per_cpu(bh_lrup, smp_processor_id()));
 	__invalidate_bh_lrus(b);
+	rcu_read_unlock();
 	bh_lru_unlock();
 }
 
@@ -2968,15 +3010,25 @@ void free_buffer_head(struct buffer_head *bh)
 }
 EXPORT_SYMBOL(free_buffer_head);
 
+static int buffer_cpu_online(unsigned int cpu)
+{
+	rcu_assign_pointer(per_cpu(bh_lrup, cpu),
+			   per_cpu_ptr(&bh_lrus[bh_lru_idx], cpu));
+	return 0;
+}
+
 static int buffer_exit_cpu_dead(unsigned int cpu)
 {
 	int i;
-	struct bh_lru *b = &per_cpu(bh_lrus, cpu);
+	struct bh_lru *b;
 
+	rcu_read_lock();
+	b = rcu_dereference(per_cpu(bh_lrup, cpu));
 	for (i = 0; i < BH_LRU_SIZE; i++) {
 		brelse(b->bhs[i]);
 		b->bhs[i] = NULL;
 	}
+	rcu_read_unlock();
 	this_cpu_add(bh_accounting.nr, per_cpu(bh_accounting, cpu).nr);
 	per_cpu(bh_accounting, cpu).nr = 0;
 	return 0;
@@ -3069,7 +3121,7 @@ EXPORT_SYMBOL(__bh_read_batch);
 void __init buffer_init(void)
 {
 	unsigned long nrpages;
-	int ret;
+	int ret, cpu;
 
 	bh_cachep = kmem_cache_create("buffer_head",
 			sizeof(struct buffer_head), 0,
@@ -3077,6 +3129,11 @@ void __init buffer_init(void)
 				SLAB_MEM_SPREAD),
 				NULL);
 
+	cpus_read_lock();
+	for_each_online_cpu(cpu)
+		rcu_assign_pointer(per_cpu(bh_lrup, cpu), per_cpu_ptr(&bh_lrus[0], cpu));
+	cpus_read_unlock();
+
 	/*
 	 * Limit the bh occupancy to 10% of ZONE_NORMAL
 	 */
@@ -3085,4 +3142,7 @@ void __init buffer_init(void)
 	ret = cpuhp_setup_state_nocalls(CPUHP_FS_BUFF_DEAD, "fs/buffer:dead",
 					NULL, buffer_exit_cpu_dead);
 	WARN_ON(ret < 0);
+	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "fs/buffer:online",
+					NULL, buffer_cpu_online);
+	WARN_ON(ret < 0);
 }