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Date:   Sun, 25 Jun 2023 11:15:01 +0800
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Subject: Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless
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From:   Qi Zheng <zhengqi.arch@bytedance.com>
To:     Dave Chinner <david@fromorbit.com>, paulmck@kernel.org
Cc:     Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        tkhai@ya.ru, roman.gushchin@linux.dev, djwong@kernel.org,
        brauner@kernel.org, tytso@mit.edu, linux-kernel@vger.kernel.org,
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        linux-nfs@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-btrfs@vger.kernel.org, RCU <rcu@vger.kernel.org>
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On 2023/6/24 19:08, Qi Zheng wrote:
> Hi Dave,
> 
> On 2023/6/24 06:19, Dave Chinner wrote:
>> On Fri, Jun 23, 2023 at 09:10:57PM +0800, Qi Zheng wrote:
>>> On 2023/6/23 14:29, Dave Chinner wrote:
>>>> On Thu, Jun 22, 2023 at 05:12:02PM +0200, Vlastimil Babka wrote:
>>>>> On 6/22/23 10:53, Qi Zheng wrote:
>>>> Yes, I suggested the IDR route because radix tree lookups under RCU
>>>> with reference counted objects are a known safe pattern that we can
>>>> easily confirm is correct or not.  Hence I suggested the unification
>>>> + IDR route because it makes the life of reviewers so, so much
>>>> easier...
>>>
>>> In fact, I originally planned to try the unification + IDR method you
>>> suggested at the beginning. But in the case of CONFIG_MEMCG disabled,
>>> the struct mem_cgroup is not even defined, and root_mem_cgroup and
>>> shrinker_info will not be allocated.  This required more code 
>>> changes, so
>>> I ended up keeping the shrinker_list and implementing the above pattern.
>>
>> Yes. Go back and read what I originally said needed to be done
>> first. In the case of CONFIG_MEMCG=n, a dummy root memcg still needs
>> to exist that holds all of the global shrinkers. Then shrink_slab()
>> is only ever passed a memcg that should be iterated.
>>
>> Yes, it needs changes external to the shrinker code itself to be
>> made to work. And even if memcg's are not enabled, we can still use
>> the memcg structures to ensure a common abstraction is used for the
>> shrinker tracking infrastructure....
> 
> Yeah, what I imagined before was to define a more concise struct
> mem_cgroup in the case of CONFIG_MEMCG=n, then allocate a dummy root
> memcg on system boot:
> 
> #ifdef !CONFIG_MEMCG
> 
> struct shrinker_info {
>      struct rcu_head rcu;
>      atomic_long_t *nr_deferred;
>      unsigned long *map;
>      int map_nr_max;
> };
> 
> struct mem_cgroup_per_node {
>      struct shrinker_info __rcu    *shrinker_info;
> };
> 
> struct mem_cgroup {
>      struct mem_cgroup_per_node *nodeinfo[];
> };
> 
> #endif
> 
> But I have a concern: if all global shrinkers are tracking with the
> info->map of root memcg, a shrinker->id needs to be assigned to them,
> which will cause info->map_nr_max to become larger than before, then
> making the traversal of info->map slower.

But most of the system is 'sb-xxx' shrinker instances, they all have
the SHRINKER_MEMCG_AWARE flag, so it should have little impact on the
speed of traversing info->map. ;)

> 
>>
>>> If the above pattern is not safe, I will go back to the unification +
>>> IDR method.
>>
>> And that is exactly how we got into this mess in the first place....
> 
> I only found one similar pattern in the kernel:
> 
> fs/smb/server/oplock.c:find_same_lease_key/smb_break_all_levII_oplock/lookup_lease_in_table
> 
> But IIUC, the refcount here needs to be decremented after holding
> rcu lock as I did above.
> 
> So regardless of whether we choose unification + IDR in the end, I still
> want to confirm whether the pattern I implemented above is safe. :)

Also + RCU mailing list.

> 
> Thanks,
> Qi
> 
>>
>> -Dave