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From: Shakeel Butt <shakeelb@google.com>
Date: Tue, 3 Jul 2018 08:46:28 -0700
Message-ID: <CALvZod7xAP9AjRWp2XX1uJBkuOprYKCf7hzAXNTdw89dc-n4OA@mail.gmail.com>
Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker
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To: Matthew Wilcox <willy@infradead.org>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>, 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>

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.

thanks,
Shakeel