On Mon, Nov 13, 2017 at 1:37 PM, Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> When shrinker_rwsem was introduced, it was assumed that
> register_shrinker()/unregister_shrinker() are really unlikely paths
> which are called during initialization and tear down. But nowadays,
> register_shrinker()/unregister_shrinker() might be called regularly.
> This patch prepares for allowing parallel registration/unregistration
> of shrinkers.
>
> Since do_shrink_slab() can reschedule, we cannot protect shrinker_list
> using one RCU section. But using atomic_inc()/atomic_dec() for each
> do_shrink_slab() call will not impact so much.
>
> This patch uses polling loop with short sleep for unregister_shrinker()
> rather than wait_on_atomic_t(), for we can save reader's cost (plain
> atomic_dec() compared to atomic_dec_and_test()), we can expect that
> do_shrink_slab() of unregistering shrinker likely returns shortly, and
> we can avoid khungtaskd warnings when do_shrink_slab() of unregistering
> shrinker unexpectedly took so long.
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reviewed-and-tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> ---
> include/linux/shrinker.h | 3 ++-
> mm/vmscan.c | 41 +++++++++++++++++++----------------------
> 2 files changed, 21 insertions(+), 23 deletions(-)
>
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 388ff29..333a1d0 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -62,9 +62,10 @@ struct shrinker {
>
> int seeks; /* seeks to recreate an obj */
> long batch; /* reclaim batch size, 0 = default */
> - unsigned long flags;
> + unsigned int flags;
>
> /* These are for internal use */
> + atomic_t nr_active;
> struct list_head list;
> /* objs pending delete, per node */
> atomic_long_t *nr_deferred;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 1c1bc95..c8996e8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -157,7 +157,7 @@ struct scan_control {
> unsigned long vm_total_pages;
>
> static LIST_HEAD(shrinker_list);
> -static DECLARE_RWSEM(shrinker_rwsem);
> +static DEFINE_MUTEX(shrinker_lock);
>
> #ifdef CONFIG_MEMCG
> static bool global_reclaim(struct scan_control *sc)
> @@ -285,9 +285,10 @@ int register_shrinker(struct shrinker *shrinker)
> if (!shrinker->nr_deferred)
> return -ENOMEM;
>
> - down_write(&shrinker_rwsem);
> - list_add_tail(&shrinker->list, &shrinker_list);
> - up_write(&shrinker_rwsem);
> + atomic_set(&shrinker->nr_active, 0);
> + mutex_lock(&shrinker_lock);
> + list_add_tail_rcu(&shrinker->list, &shrinker_list);
> + mutex_unlock(&shrinker_lock);
> return 0;
> }
> EXPORT_SYMBOL(register_shrinker);
> @@ -297,9 +298,13 @@ int register_shrinker(struct shrinker *shrinker)
> */
> void unregister_shrinker(struct shrinker *shrinker)
> {
> - down_write(&shrinker_rwsem);
> - list_del(&shrinker->list);
> - up_write(&shrinker_rwsem);
> + mutex_lock(&shrinker_lock);
> + list_del_rcu(&shrinker->list);
> + synchronize_rcu();
> + while (atomic_read(&shrinker->nr_active))
> + schedule_timeout_uninterruptible(1);
> + synchronize_rcu();
> + mutex_unlock(&shrinker_lock);
> kfree(shrinker->nr_deferred);
> }
> EXPORT_SYMBOL(unregister_shrinker);
> @@ -468,18 +473,8 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> if (nr_scanned == 0)
> nr_scanned = SWAP_CLUSTER_MAX;
>
> - if (!down_read_trylock(&shrinker_rwsem)) {
> - /*
> - * If we would return 0, our callers would understand that we
> - * have nothing else to shrink and give up trying. By returning
> - * 1 we keep it going and assume we'll be able to shrink next
> - * time.
> - */
> - freed = 1;
> - goto out;
> - }
> -
> - list_for_each_entry(shrinker, &shrinker_list, list) {
> + rcu_read_lock();
> + list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
> struct shrink_control sc = {
> .gfp_mask = gfp_mask,
> .nid = nid,
> @@ -498,11 +493,13 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> sc.nid = 0;
>
> + atomic_inc(&shrinker->nr_active);
> + rcu_read_unlock();
> freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
> + rcu_read_lock();
> + atomic_dec(&shrinker->nr_active);
> }
> -
> - up_read(&shrinker_rwsem);
> -out:
> + rcu_read_unlock();
> cond_resched();
> return freed;
> }
> --
> 1.8.3.1
>
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