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Date: Tue, 21 Jul 2015 14:19:32 +0200
From: Michal Hocko <mhocko@...nel.org>
To: Davidlohr Bueso <dave@...olabs.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>,
Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] mm: srcu-ify shrinkers
On Thu 16-07-15 13:55:13, Davidlohr Bueso wrote:
> The shrinker_rwsem is a global lock that protects the shrinker_list,
> serializing a shrinking call with register/unregistering the shrinker
> itself. As such, this lock is taken mostly for reading. In the unlikely
> case that the the list is being modified, we simply return indicating
> we want to iterate again. However, the only caller of shrink_slab()
> that acknowledges this return is drop_slab_node(), so in practice, the
> rest of the callers never try again.
>
> This patch proposes replacing the rwsem with an srcu aware list of
> shrinkers, where (un)registering tasks use a spinlock. Upon shrinker
> calls, the srcu read lock will guarantee the existence of the structure,
> thus safely optimizing the common (read locked) case. These guarantees
> also allow us to cleanup and simplify the code, getting rid of the
> ugly trylock mechanism to retry the shrinker operation when the list
> is concurrently being modified. As Michal pointed this is only worth
> mentioning for unregister purposes.
>
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Looks reasonable to me. I am pretty sure that any performance
improvements are close to 0 but the code looks better to me.
Acked-by: Michal Hocko <mhocko@...e.com>
> ---
> Changes from v1:
> - Got rid of the trylock, per mhocko.
>
> fs/super.c | 8 ++++----
> mm/vmscan.c | 39 ++++++++++++++++++---------------------
> 2 files changed, 22 insertions(+), 25 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index b613723..c917817 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -49,8 +49,8 @@ static char *sb_writers_name[SB_FREEZE_LEVELS] = {
> * One thing we have to be careful of with a per-sb shrinker is that we don't
> * drop the last active reference to the superblock from within the shrinker.
> * If that happens we could trigger unregistering the shrinker from within the
> - * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
> - * take a passive reference to the superblock to avoid this from occurring.
> + * shrinker path. Hence we take a passive reference to the superblock to avoid
> + * this from occurring.
> */
> static unsigned long super_cache_scan(struct shrinker *shrink,
> struct shrink_control *sc)
> @@ -121,8 +121,8 @@ static unsigned long super_cache_count(struct shrinker *shrink,
> * Don't call trylock_super as it is a potential
> * scalability bottleneck. The counts could get updated
> * between super_cache_count and super_cache_scan anyway.
> - * Call to super_cache_count with shrinker_rwsem held
> - * ensures the safety of call to list_lru_shrink_count() and
> + * SRCU guarantees object validity across this call -- thus
> + * it is safe to call list_lru_shrink_count() and
> * s_op->nr_cached_objects().
> */
> if (sb->s_op && sb->s_op->nr_cached_objects)
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c8d8282..fc820cf 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -36,7 +36,7 @@
> #include <linux/cpuset.h>
> #include <linux/compaction.h>
> #include <linux/notifier.h>
> -#include <linux/rwsem.h>
> +#include <linux/srcu.h>
> #include <linux/delay.h>
> #include <linux/kthread.h>
> #include <linux/freezer.h>
> @@ -146,8 +146,9 @@ int vm_swappiness = 60;
> */
> unsigned long vm_total_pages;
>
> +DEFINE_STATIC_SRCU(shrinker_srcu);
> static LIST_HEAD(shrinker_list);
> -static DECLARE_RWSEM(shrinker_rwsem);
> +static DEFINE_SPINLOCK(shrinker_list_lock);
>
> #ifdef CONFIG_MEMCG
> static bool global_reclaim(struct scan_control *sc)
> @@ -242,9 +243,9 @@ 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);
> + spin_lock(&shrinker_list_lock);
> + list_add_tail_rcu(&shrinker->list, &shrinker_list);
> + spin_unlock(&shrinker_list_lock);
> return 0;
> }
> EXPORT_SYMBOL(register_shrinker);
> @@ -254,9 +255,14 @@ EXPORT_SYMBOL(register_shrinker);
> */
> void unregister_shrinker(struct shrinker *shrinker)
> {
> - down_write(&shrinker_rwsem);
> - list_del(&shrinker->list);
> - up_write(&shrinker_rwsem);
> + spin_lock(&shrinker_list_lock);
> + list_del_rcu(&shrinker->list);
> + spin_unlock(&shrinker_list_lock);
> + /*
> + * Before freeing nr_deferred, ensure all srcu
> + * readers are done with their critical region.
> + */
> + synchronize_srcu(&shrinker_srcu);
> kfree(shrinker->nr_deferred);
> }
> EXPORT_SYMBOL(unregister_shrinker);
> @@ -408,6 +414,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> unsigned long nr_scanned,
> unsigned long nr_eligible)
> {
> + int idx;
> struct shrinker *shrinker;
> unsigned long freed = 0;
>
> @@ -417,18 +424,9 @@ 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;
> - }
> + idx = srcu_read_lock(&shrinker_srcu);
>
> - 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,
> @@ -444,8 +442,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
> }
>
> - up_read(&shrinker_rwsem);
> -out:
> + srcu_read_unlock(&shrinker_srcu, idx);
> cond_resched();
> return freed;
> }
> --
> 2.1.4
>
>
>
> --
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--
Michal Hocko
SUSE Labs
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