linux-kernel - Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless

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Date:   Thu, 23 Feb 2023 10:37:21 -0500
From:   Rafael Aquini <aquini@...hat.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     akpm@...ux-foundation.org, tkhai@...ru, hannes@...xchg.org,
        shakeelb@...gle.com, mhocko@...nel.org, roman.gushchin@...ux.dev,
        muchun.song@...ux.dev, david@...hat.com, shy828301@...il.com,
        sultan@...neltoast.com, dave@...olabs.net,
        penguin-kernel@...ove.sakura.ne.jp, paulmck@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless

On Thu, Feb 23, 2023 at 10:26:45AM -0500, Rafael Aquini wrote:
> On Thu, Feb 23, 2023 at 09:27:20PM +0800, Qi Zheng wrote:
> > The shrinker_rwsem is a global lock in shrinkers subsystem,
> > it is easy to cause blocking in the following cases:
> > 
> > a. the write lock of shrinker_rwsem was held for too long.
> >    For example, there are many memcgs in the system, which
> >    causes some paths to hold locks and traverse it for too
> >    long. (e.g. expand_shrinker_info())
> > b. the read lock of shrinker_rwsem was held for too long,
> >    and a writer came at this time. Then this writer will be
> >    forced to wait and block all subsequent readers.
> >    For example:
> >    - be scheduled when the read lock of shrinker_rwsem is
> >      held in do_shrink_slab()
> >    - some shrinker are blocked for too long. Like the case
> >      mentioned in the patchset[1].
> > 
> > Therefore, many times in history ([2],[3],[4],[5]), some
> > people wanted to replace shrinker_rwsem reader with SRCU,
> > but they all gave up because SRCU was not unconditionally
> > enabled.
> > 
> > But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"),
> > the SRCU is unconditionally enabled. So it's time to use
> > SRCU to protect readers who previously held shrinker_rwsem.
> > 
> > [1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@virtuozzo.com/
> > [2]. https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/
> > [3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/
> > [4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
> > [5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/
> > 
> > Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> > ---
> >  mm/vmscan.c | 27 +++++++++++----------------
> >  1 file changed, 11 insertions(+), 16 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9f895ca6216c..02987a6f95d1 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -202,6 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task,
> >  
> >  LIST_HEAD(shrinker_list);
> >  DECLARE_RWSEM(shrinker_rwsem);
> > +DEFINE_SRCU(shrinker_srcu);
> >  
> >  #ifdef CONFIG_MEMCG
> >  static int shrinker_nr_max;
> > @@ -706,7 +707,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
> >  void register_shrinker_prepared(struct shrinker *shrinker)
> >  {
> >  	down_write(&shrinker_rwsem);
> 
> I think you could revert the rwsem back to a simple mutex, now.
>

NVM, that's exactly what patch 7 does. :)