Re: [PATCH v4 45/48] mm: shrinker: make global slab shrink lockless
From: Dave Chinner via Virtualization <hidden>
Date: 2023-08-07 23:28:58
Also in:
dm-devel, dri-devel, kvm, linux-arm-msm, linux-bcache, linux-btrfs, linux-ext4, linux-f2fs-devel, linux-fsdevel, linux-mm, linux-nfs, linux-raid, linux-xfs, lkml, netdev, rcu, xen-devel
On Mon, Aug 07, 2023 at 07:09:33PM +0800, Qi Zheng wrote:
The shrinker_rwsem is a global read-write lock in shrinkers subsystem, which protects most operations such as slab shrink, registration and unregistration of shrinkers, etc. This can easily cause problems in the following cases.
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This commit uses the refcount+RCU method [5] proposed by Dave Chinner to re-implement the lockless global slab shrink. The memcg slab shrink is handled in the subsequent patch.
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--- include/linux/shrinker.h | 17 ++++++++++ mm/shrinker.c | 70 +++++++++++++++++++++++++++++----------- 2 files changed, 68 insertions(+), 19 deletions(-)
There's no documentation in the code explaining how the lockless shrinker algorithm works. It's left to the reader to work out how this all goes together....
quoted hunk
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h index eb342994675a..f06225f18531 100644 --- a/include/linux/shrinker.h +++ b/include/linux/shrinker.h@@ -4,6 +4,8 @@ #include <linux/atomic.h> #include <linux/types.h> +#include <linux/refcount.h> +#include <linux/completion.h> #define SHRINKER_UNIT_BITS BITS_PER_LONG@@ -87,6 +89,10 @@ struct shrinker { int seeks; /* seeks to recreate an obj */ unsigned flags; + refcount_t refcount; + struct completion done; + struct rcu_head rcu;
What does the refcount protect, why do we need the completion, etc?
quoted hunk
+ void *private_data; /* These are for internal use */@@ -120,6 +126,17 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...); void shrinker_register(struct shrinker *shrinker); void shrinker_free(struct shrinker *shrinker); +static inline bool shrinker_try_get(struct shrinker *shrinker) +{ + return refcount_inc_not_zero(&shrinker->refcount); +} + +static inline void shrinker_put(struct shrinker *shrinker) +{ + if (refcount_dec_and_test(&shrinker->refcount)) + complete(&shrinker->done); +} + #ifdef CONFIG_SHRINKER_DEBUG extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...);diff --git a/mm/shrinker.c b/mm/shrinker.c index 1911c06b8af5..d318f5621862 100644 --- a/mm/shrinker.c +++ b/mm/shrinker.c@@ -2,6 +2,7 @@ #include <linux/memcontrol.h> #include <linux/rwsem.h> #include <linux/shrinker.h> +#include <linux/rculist.h> #include <trace/events/vmscan.h> #include "internal.h"@@ -577,33 +578,42 @@ unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg, 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; - - 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, .memcg = memcg, }; + if (!shrinker_try_get(shrinker)) + continue; + + /* + * We can safely unlock the RCU lock here since we already + * hold the refcount of the shrinker. + */ + rcu_read_unlock(); + ret = do_shrink_slab(&sc, shrinker, priority); 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. + * This shrinker may be deleted from shrinker_list and freed + * after the shrinker_put() below, but this shrinker is still + * used for the next traversal. So it is necessary to hold the + * RCU lock first to prevent this shrinker from being freed, + * which also ensures that the next shrinker that is traversed + * will not be freed (even if it is deleted from shrinker_list + * at the same time). */
This comment really should be at the head of the function, describing the algorithm used within the function itself. i.e. how reference counts are used w.r.t. the rcu_read_lock() usage to guarantee existence of the shrinker and the validity of the list walk. I'm not going to remember all these little details when I look at this code in another 6 months time, and having to work it out from first principles every time I look at the code will waste of a lot of time... -Dave. -- Dave Chinner david@fromorbit.com _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization