Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless

[Qi Zheng <zhengqi.arch@bytedance.com>]

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@bytedance.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 /@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?

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