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Date: Fri, 24 Feb 2023 12:16:26 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Sultan Alsawaf <sultan@...neltoast.com>, paulmck@...nel.org
Cc: akpm@...ux-foundation.org, tkhai@...ru, 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,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
On 2023/2/24 12:00, 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?
And when I use the synchronize_srcu_expedited() mentioned by Paul,
the measured time consumption has a more obvious decrease:
@ns[umount]:
[16K, 32K) 105 |@@@@@@@@@@
|
[32K, 64K) 521
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[64K, 128K) 119 |@@@@@@@@@@@
|
[128K, 256K) 32 |@@@
|
[256K, 512K) 70 |@@@@@@
|
[512K, 1M) 49 |@@@@
|
[1M, 2M) 34 |@@@
|
[2M, 4M) 18 |@
|
[4M, 8M) 4 |
>
> Thanks,
> Qi
>
>>
>> Although I never got around to submitting it, I made a non-SRCU
>> solution [1]
>> that uses fine-grained locking instead, which is fair to both the
>> register path
>> and unregister path. (The patch I've linked is a version of this
>> adapted to an
>> older 4.14 kernel FYI, but it can be reworked for the current kernel.)
>>
>> What do you think about the fine-grained locking approach?
>>
>> Thanks,
>> Sultan
>>
>> [1]
>> https://github.com/kerneltoast/android_kernel_google_floral/commit/012378f3173a82d2333d3ae7326691544301e76a
>>
>
--
Thanks,
Qi
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