On 2023/2/25 05:14, Kirill Tkhai wrote:
On 25.02.2023 00:02, Kirill Tkhai wrote:
On 24.02.2023 07:00, Qi Zheng wrote:
On 2023/2/24 02:24, Sultan Alsawaf wrote:
On Thu, Feb 23, 2023 at 09:27:20PM +0800, Qi Zheng wrote:
The shrinker_rwsem is a global lock in shrinkers subsystem,
it is easy to cause blocking in the following cases:
a. the write lock of shrinker_rwsem was held for too long.
For example, there are many memcgs in the system, which
causes some paths to hold locks and traverse it for too
long. (e.g. expand_shrinker_info())
b. the read lock of shrinker_rwsem was held for too long,
and a writer came at this time. Then this writer will be
forced to wait and block all subsequent readers.
For example:
- be scheduled when the read lock of shrinker_rwsem is
held in do_shrink_slab()
- some shrinker are blocked for too long. Like the case
mentioned in the patchset[1].
Therefore, many times in history ([2],[3],[4],[5]), some
people wanted to replace shrinker_rwsem reader with SRCU,
but they all gave up because SRCU was not unconditionally
enabled.
But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"),
the SRCU is unconditionally enabled. So it's time to use
SRCU to protect readers who previously held shrinker_rwsem.
[1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@xxxxxxxxxxxxx/
[2]. https://lore.kernel.org/all/1437080113.3596.2.camel@xxxxxxxxxxxx/
[3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx/
[4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
[5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@xxxxxxxxxxxxxxx/
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
mm/vmscan.c | 27 +++++++++++----------------
1 file changed, 11 insertions(+), 16 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9f895ca6216c..02987a6f95d1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -202,6 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task,
LIST_HEAD(shrinker_list);
DECLARE_RWSEM(shrinker_rwsem);
+DEFINE_SRCU(shrinker_srcu);
#ifdef CONFIG_MEMCG
static int shrinker_nr_max;
@@ -706,7 +707,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
void register_shrinker_prepared(struct shrinker *shrinker)
{
down_write(&shrinker_rwsem);
- list_add_tail(&shrinker->list, &shrinker_list);
+ list_add_tail_rcu(&shrinker->list, &shrinker_list);
shrinker->flags |= SHRINKER_REGISTERED;
shrinker_debugfs_add(shrinker);
up_write(&shrinker_rwsem);
@@ -760,13 +761,15 @@ void unregister_shrinker(struct shrinker *shrinker)
return;
down_write(&shrinker_rwsem);
- list_del(&shrinker->list);
+ list_del_rcu(&shrinker->list);
shrinker->flags &= ~SHRINKER_REGISTERED;
if (shrinker->flags & SHRINKER_MEMCG_AWARE)
unregister_memcg_shrinker(shrinker);
debugfs_entry = shrinker_debugfs_remove(shrinker);
up_write(&shrinker_rwsem);
+ synchronize_srcu(&shrinker_srcu);
+
debugfs_remove_recursive(debugfs_entry);
kfree(shrinker->nr_deferred);
@@ -786,6 +789,7 @@ void synchronize_shrinkers(void)
{
down_write(&shrinker_rwsem);
up_write(&shrinker_rwsem);
+ synchronize_srcu(&shrinker_srcu);
}
EXPORT_SYMBOL(synchronize_shrinkers);
@@ -996,6 +1000,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
{
unsigned long ret, freed = 0;
struct shrinker *shrinker;
+ int srcu_idx;
/*
* The root memcg might be allocated even though memcg is disabled
@@ -1007,10 +1012,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
- if (!down_read_trylock(&shrinker_rwsem))
- goto out;
+ srcu_idx = srcu_read_lock(&shrinker_srcu);
- list_for_each_entry(shrinker, &shrinker_list, list) {
+ list_for_each_entry_srcu(shrinker, &shrinker_list, list,
+ srcu_read_lock_held(&shrinker_srcu)) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
@@ -1021,19 +1026,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (ret == SHRINK_EMPTY)
ret = 0;
freed += ret;
- /*
- * Bail out if someone want to register a new shrinker to
- * prevent the registration from being stalled for long periods
- * by parallel ongoing shrinking.
- */
- if (rwsem_is_contended(&shrinker_rwsem)) {
- freed = freed ? : 1;
- break;
- }
}
- up_read(&shrinker_rwsem);
-out:
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
cond_resched();
return freed;
}
--
2.20.1
Hi Qi,
A different problem I realized after my old attempt to use SRCU was that the
unregister_shrinker() path became quite slow due to the heavy synchronize_srcu()
call. Both register_shrinker() *and* unregister_shrinker() are called frequently
these days, and SRCU is too unfair to the unregister path IMO.
Hi Sultan,
IIUC, for unregister_shrinker(), the wait time is hardly longer with
SRCU than with shrinker_rwsem before.
And I just did a simple test. After using the script in cover letter to
increase the shrink_slab hotspot, I did umount 1k times at the same
time, and then I used bpftrace to measure the time consumption of
unregister_shrinker() as follows:
bpftrace -e 'kprobe:unregister_shrinker { @start[tid] = nsecs; } kretprobe:unregister_shrinker /@start[tid]/ { @ns[comm] = hist(nsecs - @start[tid]); delete(@start[tid]); }'
@ns[umount]:
[16K, 32K) 3 | |
[32K, 64K) 66 |@@@@@@@@@@ |
[64K, 128K) 32 |@@@@@ |
[128K, 256K) 22 |@@@ |
[256K, 512K) 48 |@@@@@@@ |
[512K, 1M) 19 |@@@ |
[1M, 2M) 131 |@@@@@@@@@@@@@@@@@@@@@ |
[2M, 4M) 313 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[4M, 8M) 302 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[8M, 16M) 55 |@@@@@@@@@
I see that the highest time-consuming of unregister_shrinker() is between 8ms and 16ms, which feels tolerable?
Hi Kirill,
The fundamental difference is that before the patchset this for_each_set_bit() iteration could be broken in the middle
of two do_shrink_slab() calls, while after the patchset we can leave for_each_set_bit() only after visiting all set bits.
After looking at the git log[1], I saw that we originally introduced
rwsem_is_contendent() here to aviod blocking register_shrinker(),
not unregister_shrinker().
So I am curious, do we really care about the speed of
unregister_shrinker()?
And after using SRCU, register_shrinker() will not be blocked by slab
shrink at all.
[1]. https://github.com/torvalds/linux/commit/e496612
Using only synchronize_srcu_expedited() won't help here.
My opinion is we should restore a check similar to the rwsem_is_contendent() check that we had before. Something like
If we really care about the speed of unregister_shrinker() like
register_shrinker(), I think this is a good idea. This guarantees
at least the speed of the unregister_shrinker() is not deteriorated. :)
the below on top of your patchset merged into appropriate patch:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 27ef9946ae8a..50e7812468ec 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -204,6 +204,7 @@ static void set_task_reclaim_state(struct task_struct *task,
LIST_HEAD(shrinker_list);
DEFINE_MUTEX(shrinker_mutex);
DEFINE_SRCU(shrinker_srcu);
+static atomic_t shrinker_srcu_generation = ATOMIC_INIT(0);
#ifdef CONFIG_MEMCG
static int shrinker_nr_max;
@@ -782,6 +783,7 @@ void unregister_shrinker(struct shrinker *shrinker)
debugfs_entry = shrinker_debugfs_remove(shrinker);
mutex_unlock(&shrinker_mutex);
+ atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
debugfs_remove_recursive(debugfs_entry);
@@ -799,6 +801,7 @@ EXPORT_SYMBOL(unregister_shrinker);
*/
void synchronize_shrinkers(void)
{
+ atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
}
EXPORT_SYMBOL(synchronize_shrinkers);
@@ -908,7 +911,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
{
struct shrinker_info *info;
unsigned long ret, freed = 0;
- int srcu_idx;
+ int srcu_idx, generation;
int i;
if (!mem_cgroup_online(memcg))
@@ -919,6 +922,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
if (unlikely(!info))
goto unlock;
+ generation = atomic_read(&shrinker_srcu_generation);
for_each_set_bit(i, info->map, info->map_nr_max) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
@@ -965,6 +969,11 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
set_shrinker_bit(memcg, nid, i);
}
freed += ret;
+
+ if (atomic_read(&shrinker_srcu_generation) != generation) {
+ freed = freed ? : 1;
+ break;
+ }
}
unlock:
srcu_read_unlock(&shrinker_srcu, srcu_idx);
@@ -1004,7 +1013,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
{
unsigned long ret, freed = 0;
struct shrinker *shrinker;
- int srcu_idx;
+ int srcu_idx, generation;
/*
* The root memcg might be allocated even though memcg is disabled
@@ -1017,6 +1026,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
srcu_idx = srcu_read_lock(&shrinker_srcu);
+ generation = atomic_read(&shrinker_srcu_generation);
list_for_each_entry_srcu(shrinker, &shrinker_list, list,
srcu_read_lock_held(&shrinker_srcu)) {
@@ -1030,6 +1040,11 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (ret == SHRINK_EMPTY)
ret = 0;
freed += ret;
+
+ if (atomic_read(&shrinker_srcu_generation) != generation) {
+ freed = freed ? : 1;
+ break;
+ }
}
srcu_read_unlock(&shrinker_srcu, srcu_idx);
Even more, for memcg shrinkers we may unlock SRCU and continue iterations from the same shrinker id:
Maybe we can also do this for global slab shrink? Like below:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index ffddbd204259..9d8c53075298 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1012,7 +1012,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask,
int nid,
int priority)
{
unsigned long ret, freed = 0;
- struct shrinker *shrinker;
+ struct shrinker *shrinker = NULL;
int srcu_idx, generation;
/*
@@ -1025,11 +1025,15 @@ static unsigned long shrink_slab(gfp_t gfp_mask,
int nid,
if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
+again:
srcu_idx = srcu_read_lock(&shrinker_srcu);
generation = atomic_read(&shrinker_srcu_generation);
- list_for_each_entry_srcu(shrinker, &shrinker_list, list,
- srcu_read_lock_held(&shrinker_srcu)) {
+ if (!shrinker)
+ shrinker = list_entry_rcu(shrinker_list.next, struct
shrinker, list);
+ else
+ shrinker = list_entry_rcu(shrinker->list.next, struct
shrinker, list);
+ list_for_each_entry_from_rcu(shrinker, &shrinker_list, list) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
@@ -1042,8 +1046,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask,
int nid,
freed += ret;
if (atomic_read(&shrinker_srcu_generation) != generation) {
- freed = freed ? : 1;
- break;
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
+ cond_resched();
+ goto again;
}
}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 27ef9946ae8a..0b197bba1257 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -204,6 +204,7 @@ static void set_task_reclaim_state(struct task_struct *task,
LIST_HEAD(shrinker_list);
DEFINE_MUTEX(shrinker_mutex);
DEFINE_SRCU(shrinker_srcu);
+static atomic_t shrinker_srcu_generation = ATOMIC_INIT(0);
#ifdef CONFIG_MEMCG
static int shrinker_nr_max;
@@ -782,6 +783,7 @@ void unregister_shrinker(struct shrinker *shrinker)
debugfs_entry = shrinker_debugfs_remove(shrinker);
mutex_unlock(&shrinker_mutex);
+ atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
debugfs_remove_recursive(debugfs_entry);
@@ -799,6 +801,7 @@ EXPORT_SYMBOL(unregister_shrinker);
*/
void synchronize_shrinkers(void)
{
+ atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
}
EXPORT_SYMBOL(synchronize_shrinkers);
@@ -908,18 +911,19 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
{
struct shrinker_info *info;
unsigned long ret, freed = 0;
- int srcu_idx;
- int i;
+ int srcu_idx, generation;
+ int i = 0;
if (!mem_cgroup_online(memcg))
return 0;
-
+again:
srcu_idx = srcu_read_lock(&shrinker_srcu);
info = shrinker_info_srcu(memcg, nid);
if (unlikely(!info))
goto unlock;
- for_each_set_bit(i, info->map, info->map_nr_max) {
+ generation = atomic_read(&shrinker_srcu_generation);
+ for_each_set_bit_from(i, info->map, info->map_nr_max) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
@@ -965,6 +969,11 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
set_shrinker_bit(memcg, nid, i);
}
freed += ret;
+
+ if (atomic_read(&shrinker_srcu_generation) != generation) {
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
Maybe we can add the following code here, so as to avoid repeating the
current id and avoid triggering softlockup:
i++;
cond_resched();
Thanks,
Qi
+ goto again;
+ }
}
unlock:
srcu_read_unlock(&shrinker_srcu, srcu_idx);
@@ -1004,7 +1013,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
{
unsigned long ret, freed = 0;
struct shrinker *shrinker;
- int srcu_idx;
+ int srcu_idx, generation;
/*
* The root memcg might be allocated even though memcg is disabled
@@ -1017,6 +1026,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
srcu_idx = srcu_read_lock(&shrinker_srcu);
+ generation = atomic_read(&shrinker_srcu_generation);
list_for_each_entry_srcu(shrinker, &shrinker_list, list,
srcu_read_lock_held(&shrinker_srcu)) {
@@ -1030,6 +1040,11 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (ret == SHRINK_EMPTY)
ret = 0;
freed += ret;
+
+ if (atomic_read(&shrinker_srcu_generation) != generation) {
+ freed = freed ? : 1;
+ break;
+ }
}
srcu_read_unlock(&shrinker_srcu, srcu_idx);
--
Thanks,
Qi
