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

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Date:   Tue, 3 Jul 2018 08:27:23 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     vdavydov.dev@...il.com, shakeelb@...gle.com,
        viro@...iv.linux.org.uk, hannes@...xchg.org, mhocko@...nel.org,
        tglx@...utronix.de, pombredanne@...b.com, stummala@...eaurora.org,
        gregkh@...uxfoundation.org, sfr@...b.auug.org.au, guro@...com,
        mka@...omium.org, penguin-kernel@...ove.SAKURA.ne.jp,
        chris@...is-wilson.co.uk, longman@...hat.com, minchan@...nel.org,
        ying.huang@...el.com, mgorman@...hsingularity.net, jbacik@...com,
        linux@...ck-us.net, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, lirongqing@...du.com, aryabinin@...tuozzo.com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

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.