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Date: Sat, 25 Feb 2023 16:08:43 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Kirill Tkhai <tkhai@...ru>, Sultan Alsawaf <sultan@...neltoast.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, shakeelb@...gle.com,
mhocko@...nel.org, roman.gushchin@...ux.dev, muchun.song@...ux.dev,
david@...hat.com, shy828301@...il.com, dave@...olabs.net,
penguin-kernel@...ove.sakura.ne.jp, paulmck@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
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@virtuozzo.com/
>>>>> [2]. https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/
>>>>> [3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/
>>>>> [4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
>>>>> [5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/
>>>>>
>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>>>>> ---
>>>>> 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 /@...rt[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
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