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@xxxxxxxxxx>
---
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);
}
