Revise ARC shrinker algorithm

The ARC shrinker callback `arc_shrinker_count/_scan()` is invoked by the
kernel's shrinker mechanism when the system is running low on free
pages.  This happens via 2 code paths:

1. "direct reclaim": The system is attempting to allocate a page, but we
are low on memory.  The ARC shrinker callback is invoked from the
page-allocation code path.

2. "indirect reclaim": kswapd notices that there aren't many free pages,
so it invokes the ARC shrinker callback.

In both cases, the kernel's shrinker code requests that the ARC shrinker
callback release some of its cache, and then it measures how many pages
were released.  However, it's measurement of released pages does not
include pages that are freed via `__free_pages()`, which is how the ARC
releases memory (via `abd_free_chunks()`).  Rather, the kernel shrinker
code is looking for pages to be placed on the lists of reclaimable pages
(which is separate from actually-free pages).

Because the kernel shrinker code doesn't detect that the ARC has
released pages, it may call the ARC shrinker callback many times,
resulting in the ARC "collapsing" down to `arc_c_min`.  This has several
negative impacts:

1. ZFS doesn't use RAM to cache data effectively.

2. In the direct reclaim case, a single page allocation may wait a long
time (e.g. more than a minute) while we evict the entire ARC.

3. Even with the improvements made in 67c0f0dedc5 ("ARC shrinking blocks
reads/writes"), occasionally `arc_size` may stay above `arc_c` for the
entire time of the ARC collapse, thus blocking ZFS read/write operations
in `arc_get_data_impl()`.

To address these issues, this commit limits the ways that the ARC
shrinker callback can be used by the kernel shrinker code, and mitigates
the impact of arc_is_overflowing() on ZFS read/write operations.

With this commit:

1. We limit the amount of data that can be reclaimed from the ARC via
the "direct reclaim" shrinker.  This limits the amount of time it takes
to allocate a single page.

2. We do not allow the ARC to shrink via kswapd (indirect reclaim).
Instead we rely on `arc_evict_zthr` to monitor free memory and reduce
the ARC target size to keep sufficient free memory in the system.  Note
that we can't simply rely on limiting the amount that we reclaim at once
(as for the direct reclaim case), because kswapd's "boosted" logic can
invoke the callback an unlimited number of times (see
`balance_pgdat()`).

3. When `arc_is_overflowing()` and we want to allocate memory,
`arc_get_data_impl()` will wait only for a multiple of the requested
amount of data to be evicted, rather than waiting for the ARC to no
longer be overflowing.  This allows ZFS reads/writes to make progress
even while the ARC is overflowing, while also ensuring that the eviction
thread makes progress towards reducing the total amount of memory used
by the ARC.

4. The amount of memory that the ARC always tries to keep free for the
rest of the system, `arc_sys_free` is increased.

5. Now that the shrinker callback is able to provide feedback to the
kernel's shrinker code about our progress, we can safely enable
the kswapd hook. This will allow the arc to receive notifications
when memory pressure is first detected by the kernel. We also
re-enable the appropriate kstats to track these callbacks.

Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Co-authored-by: George Wilson <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10600 

1 files changed, 6 insertions, 14 deletions

diff --git a/include/sys/arc_impl.h b/include/sys/arc_impl.h
index eb90d5bc9..bb9163ba7 100644
--- a/include/sys/arc_impl.h
+++ b/include/sys/arc_impl.h
@@ -846,15 +846,11 @@ typedef struct arc_stats {
 	kstat_named_t arcstat_cached_only_in_progress;
 } arc_stats_t;
 
-typedef enum free_memory_reason_t {
-	FMR_UNKNOWN,
-	FMR_NEEDFREE,
-	FMR_LOTSFREE,
-	FMR_SWAPFS_MINFREE,
-	FMR_PAGES_PP_MAXIMUM,
-	FMR_HEAP_ARENA,
-	FMR_ZIO_ARENA,
-} free_memory_reason_t;
+typedef struct arc_evict_waiter {
+	list_node_t aew_node;
+	kcondvar_t aew_cv;
+	uint64_t aew_count;
+} arc_evict_waiter_t;
 
 #define	ARCSTAT(stat)	(arc_stats.stat.value.ui64)
 
@@ -870,7 +866,6 @@ typedef enum free_memory_reason_t {
 #define	arc_c_min	ARCSTAT(arcstat_c_min)	/* min target cache size */
 #define	arc_c_max	ARCSTAT(arcstat_c_max)	/* max target cache size */
 #define	arc_sys_free	ARCSTAT(arcstat_sys_free) /* target system free bytes */
-#define	arc_need_free	ARCSTAT(arcstat_need_free) /* bytes to be freed */
 
 extern taskq_t *arc_prune_taskq;
 extern arc_stats_t arc_stats;
@@ -879,10 +874,6 @@ extern boolean_t arc_warm;
 extern int arc_grow_retry;
 extern int arc_no_grow_shift;
 extern int arc_shrink_shift;
-extern zthr_t		*arc_evict_zthr;
-extern kmutex_t		arc_evict_lock;
-extern kcondvar_t	arc_evict_waiters_cv;
-extern boolean_t	arc_evict_needed;
 extern kmutex_t arc_prune_mtx;
 extern list_t arc_prune_list;
 extern aggsum_t arc_size;
@@ -897,6 +888,7 @@ extern void arc_reduce_target_size(int64_t to_free);
 extern boolean_t arc_reclaim_needed(void);
 extern void arc_kmem_reap_soon(void);
 extern boolean_t arc_is_overflowing(void);
+extern void arc_wait_for_eviction(uint64_t);
 
 extern void arc_lowmem_init(void);
 extern void arc_lowmem_fini(void);