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Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker
To:     Shakeel Butt <shakeelb@google.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Vladimir Davydov <vdavydov.dev@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        stummala@codeaurora.org, gregkh@linuxfoundation.org,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Roman Gushchin <guro@fb.com>, mka@chromium.org,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Chris Wilson <chris@chris-wilson.co.uk>, longman@redhat.com,
        Minchan Kim <minchan@kernel.org>,
        Huang Ying <ying.huang@intel.com>,
        Mel Gorman <mgorman@techsingularity.net>, jbacik@fb.com,
        Guenter Roeck <linux@roeck-us.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, lirongqing@baidu.com,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Tue, 3 Jul 2018 19:17:14 +0300
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Hi, Shakeel,

On 03.07.2018 18:46, Shakeel Butt wrote:
> On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox <willy@infradead.org> 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