Emergency slab objects

This patch is designed to resolve a deadlock which can occur with
__vmalloc() based slabs.  The issue is that the Linux kernel does
not honor the flags passed to __vmalloc().  This makes it unsafe
to use in a writeback context.  Unfortunately, this is a use case
ZFS depends on for correct operation.

Fixing this issue in the upstream kernel was pursued and patches
are available which resolve the issue.

  https://bugs.gentoo.org/show_bug.cgi?id=416685

However, these changes were rejected because upstream felt that
using __vmalloc() in the context of writeback should never be done.
Their solution was for us to rewrite parts of ZFS to accomidate
the Linux VM.

While that is probably the right long term solution, and it is
something we want to pursue, it is not a trivial task and will
likely destabilize the existing code.  This work has been planned
for the 0.7.0 release but in the meanwhile we want to improve the
SPL slab implementation to accomidate this expected ZFS usage.

This is accomplished by performing the __vmalloc() asynchronously
in the context of a work queue.  This doesn't prevent the posibility
of the worker thread from deadlocking.  However, the caller can now
safely block on a wait queue for the slab allocation to complete.

Normally this will occur in a reasonable amount of time and the
caller will be woken up when the new slab is available,.  The objects
will then get cached in the per-cpu magazines and everything will
proceed as usual.

However, if the __vmalloc() deadlocks for the reasons described
above, or is just very slow, then the callers on the wait queues
will timeout out.  When this rare situation occurs they will attempt
to kmalloc() a single minimally sized object using the GFP_NOIO flags.
This allocation will not deadlock because kmalloc() will honor the
passed flags and the caller will be able to make forward progress.

As long as forward progress can be maintained then even if the
worker thread is deadlocked the critical thread will make progress.
This will eventually allow the deadlocked worker thread to complete
and normal operation will resume.

These emergency allocations will likely be slow since they require
contiguous pages.  However, their use should be rare so the impact
is expected to be minimal.  If that turns out not to be the case in
practice further optimizations are possible.

One additional concern is if these emergency objects are long lived.
Right now they are simply tracked on a list which must be walked when
an object is freed.  Is they accumulate on a system and the list
grows freeing objects will become more expensive.  This could be
handled relatively easily by using a hash instead of a list, but that
optimization (if needed) is left for a follow up patch.

Additionally, these emeregency objects could be repacked in to existing
slabs as objects are freed if the kmem_cache_set_move() functionality
was implemented.  See issue https://github.com/zfsonlinux/spl/issues/26
for full details.  This work would also help reduce ZFS's memory
fragmentation problems.

The /proc/spl/kmem/slab file has had two new columns added at the
end.  The 'emerg' column reports the current number of these emergency
objects in use for the cache, and the following 'max' column shows
the historical worst case.  These value should give us a good idea
of how often these objects are needed.  Based on these values under
real use cases we can tune the default behavior.

Lastly, as a side benefit using a single work queue for the slab
allocations should reduce cpu contention on the global virtual address
space lock.   This should manifest itself as reduced cpu usage for
the system.

Signed-off-by: Brian Behlendorf <[email protected]>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/sys/kmem.h b/include/sys/kmem.h
index 344e2716b..aaff6d046 100644
--- a/include/sys/kmem.h
+++ b/include/sys/kmem.h
@@ -291,6 +291,7 @@ enum {
 	KMC_BIT_KMEM		= 5,	/* Use kmem cache */
 	KMC_BIT_VMEM		= 6,	/* Use vmem cache */
 	KMC_BIT_OFFSLAB		= 7,	/* Objects not on slab */
+	KMC_BIT_GROWING         = 15,   /* Growing in progress */
 	KMC_BIT_REAPING		= 16,	/* Reaping in progress */
 	KMC_BIT_DESTROY		= 17,	/* Destroy in progress */
 	KMC_BIT_TOTAL		= 18,	/* Proc handler helper bit */
@@ -315,6 +316,7 @@ typedef enum kmem_cbrc {
 #define KMC_KMEM		(1 << KMC_BIT_KMEM)
 #define KMC_VMEM		(1 << KMC_BIT_VMEM)
 #define KMC_OFFSLAB		(1 << KMC_BIT_OFFSLAB)
+#define KMC_GROWING		(1 << KMC_BIT_GROWING)
 #define KMC_REAPING		(1 << KMC_BIT_REAPING)
 #define KMC_DESTROY		(1 << KMC_BIT_DESTROY)
 #define KMC_TOTAL		(1 << KMC_BIT_TOTAL)
@@ -374,6 +376,17 @@ typedef struct spl_kmem_slab {
 	uint32_t		sks_ref;	/* Ref count used objects */
 } spl_kmem_slab_t;
 
+typedef struct spl_kmem_alloc {
+	struct spl_kmem_cache	*ska_cache;	/* Owned by cache */
+	int			ska_flags;	/* Allocation flags */
+	struct delayed_work	ska_work;	/* Allocation work */
+} spl_kmem_alloc_t;
+
+typedef struct spl_kmem_emergency {
+	void			*ske_obj;	/* Buffer address */
+	struct list_head	ske_list;	/* Emergency list linkage */
+} spl_kmem_emergency_t;
+
 typedef struct spl_kmem_cache {
 	uint32_t		skc_magic;	/* Sanity magic */
 	uint32_t		skc_name_size;	/* Name length */
@@ -398,7 +411,9 @@ typedef struct spl_kmem_cache {
 	struct list_head	skc_list;	/* List of caches linkage */
 	struct list_head	skc_complete_list;/* Completely alloc'ed */
 	struct list_head	skc_partial_list; /* Partially alloc'ed */
+	struct list_head	skc_emergency_list; /* Min sized objects */
 	spinlock_t		skc_lock;	/* Cache lock */
+	wait_queue_head_t	skc_waitq;	/* Allocation waiters */
 	uint64_t		skc_slab_fail;	/* Slab alloc failures */
 	uint64_t		skc_slab_create;/* Slab creates */
 	uint64_t		skc_slab_destroy;/* Slab destroys */
@@ -408,6 +423,8 @@ typedef struct spl_kmem_cache {
 	uint64_t		skc_obj_total;	/* Obj total current */
 	uint64_t		skc_obj_alloc;	/* Obj alloc current */
 	uint64_t		skc_obj_max;	/* Obj max historic */
+	uint64_t		skc_obj_emergency; /* Obj emergency current */
+	uint64_t		skc_obj_emergency_max; /* Obj emergency max */
 } spl_kmem_cache_t;
 #define kmem_cache_t		spl_kmem_cache_t