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Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker
To:     Matthew Wilcox <willy@infradead.org>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Tue, 3 Jul 2018 18:46:57 +0300
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On 03.07.2018 18:27, Matthew Wilcox 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.

shrinker_idr now contains only memcg-aware shrinkers, so all bits from memcg map
may be potentially populated. In case of memcg-aware shrinkers and !memcg-aware
shrinkers share the same numbers like you suggest, this will lead to increasing
size of memcg maps, which is bad for memory consumption. So, memcg-aware shrinkers
should to have its own IDR and its own numbers. The tricks like allocation big
IDs for !memcg-aware shrinkers seem bad for me, since they make the code more
complicated.

> 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.

This patchset does not make the cache-efficient bad, since without the patchset the situation
is so bad, that it's just impossible to talk about the cache efficiently,
so let's leave lockless iteration/etc for the future works.

Kirill