Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

[Kirill Tkhai]

On 03.07.2018 20:00, Shakeel Butt wrote:
> On Tue, Jul 3, 2018 at 9:17 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>>
>> Hi, Shakeel,
>>
>> On 03.07.2018 18:46, Shakeel Butt wrote:
>>> On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>>>>
>>>> On Tue, Jul 03, 2018 at 06:09:05PM +0300, Kirill Tkhai wrote:
>>>>> +++ b/mm/vmscan.c
>>>>> @@ -169,6 +169,49 @@ unsigned long vm_total_pages;
>>>>>  static LIST_HEAD(shrinker_list);
>>>>>  static DECLARE_RWSEM(shrinker_rwsem);
>>>>>
>>>>> +#ifdef CONFIG_MEMCG_KMEM
>>>>> +static DEFINE_IDR(shrinker_idr);
>>>>> +static int shrinker_nr_max;
>>>>
>>>> So ... we've now got a list_head (shrinker_list) which contains all of
>>>> the shrinkers, plus a shrinker_idr which contains the memcg-aware shrinkers?
>>>>
>>>> Why not replace the shrinker_list with the shrinker_idr?  It's only used
>>>> twice in vmscan.c:
>>>>
>>>> void register_shrinker_prepared(struct shrinker *shrinker)
>>>> {
>>>>         down_write(&shrinker_rwsem);
>>>>         list_add_tail(&shrinker->list, &shrinker_list);
>>>>         up_write(&shrinker_rwsem);
>>>> }
>>>>
>>>>         list_for_each_entry(shrinker, &shrinker_list, list) {
>>>> ...
>>>>
>>>> The first is simply idr_alloc() and the second is
>>>>
>>>>         idr_for_each_entry(&shrinker_idr, shrinker, id) {
>>>>
>>>> I understand there's a difference between allocating the shrinker's ID and
>>>> adding it to the list.  You can do this by calling idr_alloc with NULL
>>>> as the pointer, and then using idr_replace() when you want to add the
>>>> shrinker to the list.  idr_for_each_entry() skips over NULL entries.
>>>>
>>>> This will actually reduce the size of each shrinker and be more
>>>> cache-efficient when calling the shrinkers.  I think we can also get
>>>> rid of the shrinker_rwsem eventually, but let's leave it for now.
>>>
>>> Can you explain how you envision shrinker_rwsem can be removed? I am
>>> very much interested in doing that.
>>
>> Have you tried to do some games with SRCU? It looks like we just need to
>> teach count_objects() and scan_objects() to work with semi-destructed
>> shrinkers. Though, this looks this will make impossible to introduce
>> shrinkers, which do synchronize_srcu() in scan_objects() for example.
>> Not sure, someone will actually use this, and this is possible to consider
>> as limitation.
>>
> 
> Hi Kirill, I tried SRCU and the discussion is at
> https://lore.kernel.org/lkml/20171117173521.GA21692@xxxxxxxxxxxxx/T/#u
> 
> Paul E. McKenney suggested to enable SRCU unconditionally. So, to use
> SRCU for shrinkers, we first have to push unconditional SRCU.

First time, I read this, I though the talk goes about some new srcu_read_lock()
without an argument and it's need to rework SRCU in some huge way. Thanks
god, it was just a misreading :)
> Tetsuo had another lockless solution which was a bit involved but does
> not depend on SRCU.

Ok, I see refcounters suggestion. Thanks for the link, Shakeel!

Kirill