On Thu, Feb 23, 2023 at 10:24:47AM -0800, Sultan Alsawaf 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);
> > - list_add_tail(&shrinker->list, &shrinker_list);
> > + list_add_tail_rcu(&shrinker->list, &shrinker_list);
> > shrinker->flags |= SHRINKER_REGISTERED;
> > shrinker_debugfs_add(shrinker);
> > up_write(&shrinker_rwsem);
> > @@ -760,13 +761,15 @@ void unregister_shrinker(struct shrinker *shrinker)
> > return;
> >
> > down_write(&shrinker_rwsem);
> > - list_del(&shrinker->list);
> > + list_del_rcu(&shrinker->list);
> > shrinker->flags &= ~SHRINKER_REGISTERED;
> > if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > unregister_memcg_shrinker(shrinker);
> > debugfs_entry = shrinker_debugfs_remove(shrinker);
> > up_write(&shrinker_rwsem);
> >
> > + synchronize_srcu(&shrinker_srcu);
> > +
> > debugfs_remove_recursive(debugfs_entry);
> >
> > kfree(shrinker->nr_deferred);
> > @@ -786,6 +789,7 @@ void synchronize_shrinkers(void)
> > {
> > down_write(&shrinker_rwsem);
> > up_write(&shrinker_rwsem);
> > + synchronize_srcu(&shrinker_srcu);
> > }
> > EXPORT_SYMBOL(synchronize_shrinkers);
> >
> > @@ -996,6 +1000,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > {
> > unsigned long ret, freed = 0;
> > struct shrinker *shrinker;
> > + int srcu_idx;
> >
> > /*
> > * The root memcg might be allocated even though memcg is disabled
> > @@ -1007,10 +1012,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
> > return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
> >
> > - if (!down_read_trylock(&shrinker_rwsem))
> > - goto out;
> > + srcu_idx = srcu_read_lock(&shrinker_srcu);
> >
> > - list_for_each_entry(shrinker, &shrinker_list, list) {
> > + list_for_each_entry_srcu(shrinker, &shrinker_list, list,
> > + srcu_read_lock_held(&shrinker_srcu)) {
> > struct shrink_control sc = {
> > .gfp_mask = gfp_mask,
> > .nid = nid,
> > @@ -1021,19 +1026,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > if (ret == SHRINK_EMPTY)
> > ret = 0;
> > freed += ret;
> > - /*
> > - * Bail out if someone want to register a new shrinker to
> > - * prevent the registration from being stalled for long periods
> > - * by parallel ongoing shrinking.
> > - */
> > - if (rwsem_is_contended(&shrinker_rwsem)) {
> > - freed = freed ? : 1;
> > - break;
> > - }
> > }
> >
> > - up_read(&shrinker_rwsem);
> > -out:
> > + srcu_read_unlock(&shrinker_srcu, srcu_idx);
> > cond_resched();
> > return freed;
> > }
> > --
> > 2.20.1
> >
> >
>
> Hi Qi,
>
> A different problem I realized after my old attempt to use SRCU was that the
> unregister_shrinker() path became quite slow due to the heavy synchronize_srcu()
> call. Both register_shrinker() *and* unregister_shrinker() are called frequently
> these days, and SRCU is too unfair to the unregister path IMO.
>
> Although I never got around to submitting it, I made a non-SRCU solution [1]
> that uses fine-grained locking instead, which is fair to both the register path
> and unregister path. (The patch I've linked is a version of this adapted to an
> older 4.14 kernel FYI, but it can be reworked for the current kernel.)
>
> What do you think about the fine-grained locking approach?
Another approach is to use synchronize_srcu_expedited(), which avoids
the sleeps that are otherwise used to encourage sharing of grace periods
among concurrent requests. It might be possible to use call_srcu(),
but I don't claim to know the shrinker code well enough to say for sure.
Thanx, Paul
> Thanks,
> Sultan
>
> [1] https://github.com/kerneltoast/android_kernel_google_floral/commit/012378f3173a82d2333d3ae7326691544301e76a
