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

[Kirill Tkhai]

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.

Kirill