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Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> In our production, we have observed that the job loader gets stuck for
> 10s of seconds while doing mount operation. It turns out that it was
> stuck in register_shrinker() and some unrelated job was under memory
> pressure and spending time in shrink_slab(). Our machines have a lot
> of shrinkers registered and jobs under memory pressure has to traverse
> all of those memcg-aware shrinkers and do affect unrelated jobs which
> want to register their own shrinkers.
>
> This patch has made the shrinker_list traversal lockless and shrinker
> register remain fast. For the shrinker unregister, atomic counter
> has been introduced to avoid synchronize_rcu() call. The fields of
> struct shrinker has been rearraged to make sure that the size does
> not increase for x86_64.
>
> The shrinker functions are allowed to reschedule() and thus can not
> be called with rcu read lock. One way to resolve that is to use
> srcu read lock but then ifdefs has to be used as SRCU is behind
> CONFIG_SRCU. Another way is to just release the rcu read lock before
> calling the shrinker and reacquire on the return. The atomic counter
> will make sure that the shrinker entry will not be freed under us.
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> ---
> Changelog since v1:
> - release and reacquire rcu lock across shrinker call.
>
> include/linux/shrinker.h | 4 +++-
> mm/vmscan.c | 54 ++++++++++++++++++++++++++++++------------------
> 2 files changed, 37 insertions(+), 21 deletions(-)
>
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 388ff2936a87..434b76ef9367 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -60,14 +60,16 @@ struct shrinker {
> unsigned long (*scan_objects)(struct shrinker *,
> struct shrink_control *sc);
>
> + unsigned int flags;
> int seeks; /* seeks to recreate an obj */
> long batch; /* reclaim batch size, 0 = default */
> - unsigned long flags;
>
> /* These are for internal use */
> struct list_head list;
> /* objs pending delete, per node */
> atomic_long_t *nr_deferred;
> + /* Number of active do_shrink_slab calls to this shrinker */
> + atomic_t nr_active;
> };
> #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eb2f0315b8c0..6cec46ac6d95 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -157,7 +157,7 @@ int vm_swappiness = 60;
> unsigned long vm_total_pages;
>
> static LIST_HEAD(shrinker_list);
> -static DECLARE_RWSEM(shrinker_rwsem);
> +static DEFINE_SPINLOCK(shrinker_lock);
>
> #ifdef CONFIG_MEMCG
> static bool global_reclaim(struct scan_control *sc)
> @@ -285,21 +285,42 @@ 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);
> + spin_lock(&shrinker_lock);
> + list_add_tail_rcu(&shrinker->list, &shrinker_list);
> + spin_unlock(&shrinker_lock);
> return 0;
> }
> EXPORT_SYMBOL(register_shrinker);
>
> +static void get_shrinker(struct shrinker *shrinker)
> +{
> + atomic_inc(&shrinker->nr_active);
> + rcu_read_unlock();
> +}
> +
> +static void put_shrinker(struct shrinker *shrinker)
> +{
> + rcu_read_lock();
> + if (!atomic_dec_return(&shrinker->nr_active))
> + wake_up_atomic_t(&shrinker->nr_active);
> +}
> +
> +static int shrinker_wait_atomic_t(atomic_t *p)
> +{
> + schedule();
> + return 0;
> +}
> /*
> * Remove one
> */
> void unregister_shrinker(struct shrinker *shrinker)
> {
> - down_write(&shrinker_rwsem);
> - list_del(&shrinker->list);
> - up_write(&shrinker_rwsem);
> + spin_lock(&shrinker_lock);
> + list_del_rcu(&shrinker->list);
> + spin_unlock(&shrinker_lock);
> + wait_on_atomic_t(&shrinker->nr_active, shrinker_wait_atomic_t,
> + TASK_UNINTERRUPTIBLE);
What keeps us from returning to the caller which could kfree the
shrinker before shrink_slab() uses it for list iteration?
> kfree(shrinker->nr_deferred);
> }
> EXPORT_SYMBOL(unregister_shrinker);
> @@ -468,18 +489,9 @@ 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;
> - }
> + rcu_read_lock();
>
> - list_for_each_entry(shrinker, &shrinker_list, list) {
> + list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
> struct shrink_control sc = {
> .gfp_mask = gfp_mask,
> .nid = nid,
> @@ -498,11 +510,13 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> sc.nid = 0;
>
> + get_shrinker(shrinker);
> freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
> + put_shrinker(shrinker);
> }
>
> - up_read(&shrinker_rwsem);
> -out:
> + rcu_read_unlock();
> +
> cond_resched();
> return freed;
> }
> --
> 2.15.0.403.gc27cc4dac6-goog
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