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);
Kirill
