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

[Qi Zheng]

On 2023/2/23 23:37, Rafael Aquini wrote:
> 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@xxxxxxxxxxxxx/
> > > [2]. https://lore.kernel.org/all/1437080113.3596.2.camel@xxxxxxxxxxxx/
> > > [3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx/
> > > [4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
> > > [5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@xxxxxxxxxxxxxxx/
> > >
> > > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > > ---
> > >   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. :)

Yeah. :)

>   

--
Thanks,
Qi