Remove skc_reclaim, hdr_recl, kmem_cache shrinker

The SPL kmem_cache implementation provides a mechanism, `skc_reclaim`,
whereby individual caches can register a callback to be invoked when
there is memory pressure.  This mechanism is used in only one place: the
ARC registers the `hdr_recl()` reclaim function.  This function wakes up
the `arc_reap_zthr`, whose job is to call `kmem_cache_reap()` and
`arc_reduce_target_size()`.

The `skc_reclaim` callbacks are invoked only by shrinker callbacks and
`arc_reap_zthr`, and only callback only wakes up `arc_reap_zthr`.  When
called from `arc_reap_zthr`, waking `arc_reap_zthr` is a no-op.  When
called from shrinker callbacks, we are already aware of memory pressure
and responding to it.  Therefore there is little benefit to ever calling
the `hdr_recl()` `skc_reclaim` callback.

The `arc_reap_zthr` also wakes once a second, and if memory is low when
allocating an ARC buffer.  Therefore, additionally waking it from the
shrinker calbacks has little benefit.

The shrinker callbacks can be invoked very frequently, e.g. 10,000 times
per second.  Additionally, for invocation of the shrinker callback,
skc_reclaim is invoked many times.  Therefore, this mechanism consumes
significant amounts of CPU time.

The kmem_cache shrinker calls `spl_kmem_cache_reap_now()`, which,
in addition to invoking `skc_reclaim()`, does two things to attempt to
free pages for use by the system:
 1. Return free objects from the magazine layer to the slab layer
 2. Return entirely-free slabs to the page layer (i.e. free pages)

These actions apply only to caches implemented by the SPL, not those
that use the underlying kernel SLAB/SLUB caches.  The SPL caches are
used for objects >=32KB, which are primarily linear ABD's cached in the
DBUF cache.

These actions (freeing objects from the magazine layer and returning
entirely-free slabs) are also taken whenever a `kmem_cache_free()` call
finds a full magazine.  So there would typically be zero entirely-free
slabs, and the number of objects in magazines is limited (typically no
more than 64 objects per magazine, and there's one magazine per CPU).
Therefore the benefit of `spl_kmem_cache_reap_now()`, while nonzero, is
modest.

We also call `spl_kmem_cache_reap_now()` from the `arc_reap_zthr`, when
memory pressure is detected.  Therefore, calling
`spl_kmem_cache_reap_now()` from the kmem_cache shrinker is not needed.

This commit removes the `skc_reclaim` mechanism, its only callback
`hdr_recl()`, and the kmem_cache shrinker callback.

Reviewed-By: Brian Behlendorf <[email protected]>
Reviewed-by: George Wilson <[email protected]>
Reviewed-by: Pavel Zakharov <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10576 

1 files changed, 1 insertions, 3 deletions

diff --git a/include/os/linux/spl/sys/kmem_cache.h b/include/os/linux/spl/sys/kmem_cache.h
index ce18eb474..30d2dd5ed 100644
--- a/include/os/linux/spl/sys/kmem_cache.h
+++ b/include/os/linux/spl/sys/kmem_cache.h
@@ -124,7 +124,6 @@ extern struct rw_semaphore spl_kmem_cache_sem;
 
 typedef int (*spl_kmem_ctor_t)(void *, void *, int);
 typedef void (*spl_kmem_dtor_t)(void *, void *);
-typedef void (*spl_kmem_reclaim_t)(void *);
 
 typedef struct spl_kmem_magazine {
 	uint32_t		skm_magic;	/* Sanity magic */
@@ -174,7 +173,6 @@ typedef struct spl_kmem_cache {
 	uint32_t		skc_mag_refill;	/* Magazine refill count */
 	spl_kmem_ctor_t		skc_ctor;	/* Constructor */
 	spl_kmem_dtor_t		skc_dtor;	/* Destructor */
-	spl_kmem_reclaim_t	skc_reclaim;	/* Reclaimator */
 	void			*skc_private;	/* Private data */
 	void			*skc_vmp;	/* Unused */
 	struct kmem_cache	*skc_linux_cache; /* Linux slab cache if used */
@@ -210,7 +208,7 @@ typedef struct spl_kmem_cache {
 
 extern spl_kmem_cache_t *spl_kmem_cache_create(char *name, size_t size,
     size_t align, spl_kmem_ctor_t ctor, spl_kmem_dtor_t dtor,
-    spl_kmem_reclaim_t reclaim, void *priv, void *vmp, int flags);
+    void *reclaim, void *priv, void *vmp, int flags);
 extern void spl_kmem_cache_set_move(spl_kmem_cache_t *,
     kmem_cbrc_t (*)(void *, void *, size_t, void *));
 extern void spl_kmem_cache_destroy(spl_kmem_cache_t *skc);