Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
From: Kirill Tkhai
Date: Fri Feb 24 2023 - 16:15:22 EST
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?
>
> 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.
>
> 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
> 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:
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);
+ 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);