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

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Date:   Tue, 3 Jul 2018 08:46:28 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        stummala@...eaurora.org, gregkh@...uxfoundation.org,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Roman Gushchin <guro@...com>, mka@...omium.org,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Chris Wilson <chris@...is-wilson.co.uk>, longman@...hat.com,
        Minchan Kim <minchan@...nel.org>,
        Huang Ying <ying.huang@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>, jbacik@...com,
        Guenter Roeck <linux@...ck-us.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, lirongqing@...du.com,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox <willy@...radead.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.

thanks,
Shakeel