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. :)
