Hey Mav, this seems like an important one to get in for 11.2 so just wanted to check if that was your intention as there's no MFC tag on the commit?

On 16/04/2018 01:54, Alexander Motin wrote:
Author: mav
Date: Mon Apr 16 00:54:58 2018
New Revision: 332523
URL: https://svnweb.freebsd.org/changeset/base/332523

Log:
   9433 Fix ARC hit rate
When the compressed ARC feature was added in commit d3c2ae1
   the method of reference counting in the ARC was modified.  As
   part of this accounting change the arc_buf_add_ref() function
   was removed entirely.
This would have be fine but the arc_buf_add_ref() function
   served a second undocumented purpose of updating the ARC access
   information when taking a hold on a dbuf.  Without this logic
   in place a cached dbuf would not migrate its associated
   arc_buf_hdr_t to the MFU list.  This would negatively impact
   the ARC hit rate, particularly on systems with a small ARC.
This change reinstates the missing call to arc_access() from
   dbuf_hold() by implementing a new arc_buf_access() function.
Reviewed-by: Giuseppe Di Natale <dinata...@llnl.gov>
   Reviewed-by: Tony Hutter <hutt...@llnl.gov>
   Reviewed-by: Tim Chase <t...@chase2k.com>
   Reviewed by: George Wilson <george.wil...@delphix.com>
   Reviewed-by: George Melikov <m...@gmelikov.ru>
   Signed-off-by: Brian Behlendorf <behlendo...@llnl.gov>

Modified:
   head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
   head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dbuf.c
   head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/arc.h

Modified: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
==============================================================================
--- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c   Mon Apr 16 
00:42:45 2018        (r332522)
+++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c   Mon Apr 16 
00:54:58 2018        (r332523)
@@ -540,8 +540,13 @@ typedef struct arc_stats {
         */
        kstat_named_t arcstat_mutex_miss;
        /*
+        * Number of buffers skipped when updating the access state due to the
+        * header having already been released after acquiring the hash lock.
+        */
+       kstat_named_t arcstat_access_skip;
+       /*
         * Number of buffers skipped because they have I/O in progress, are
-        * indrect prefetch buffers that have not lived long enough, or are
+        * indirect prefetch buffers that have not lived long enough, or are
         * not from the spa we're trying to evict from.
         */
        kstat_named_t arcstat_evict_skip;
@@ -796,6 +801,7 @@ static arc_stats_t arc_stats = {
        { "allocated",                        KSTAT_DATA_UINT64 },
        { "deleted",                  KSTAT_DATA_UINT64 },
        { "mutex_miss",                       KSTAT_DATA_UINT64 },
+       { "access_skip",              KSTAT_DATA_UINT64 },
        { "evict_skip",                       KSTAT_DATA_UINT64 },
        { "evict_not_enough",         KSTAT_DATA_UINT64 },
        { "evict_l2_cached",          KSTAT_DATA_UINT64 },
@@ -5063,6 +5069,51 @@ arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
        } else {
                ASSERT(!"invalid arc state");
        }
+}
+
+/*
+ * This routine is called by dbuf_hold() to update the arc_access() state
+ * which otherwise would be skipped for entries in the dbuf cache.
+ */
+void
+arc_buf_access(arc_buf_t *buf)
+{
+       mutex_enter(&buf->b_evict_lock);
+       arc_buf_hdr_t *hdr = buf->b_hdr;
+
+       /*
+        * Avoid taking the hash_lock when possible as an optimization.
+        * The header must be checked again under the hash_lock in order
+        * to handle the case where it is concurrently being released.
+        */
+       if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
+               mutex_exit(&buf->b_evict_lock);
+               ARCSTAT_BUMP(arcstat_access_skip);
+               return;
+       }
+
+       kmutex_t *hash_lock = HDR_LOCK(hdr);
+       mutex_enter(hash_lock);
+
+       if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
+               mutex_exit(hash_lock);
+               mutex_exit(&buf->b_evict_lock);
+               ARCSTAT_BUMP(arcstat_access_skip);
+               return;
+       }
+
+       mutex_exit(&buf->b_evict_lock);
+
+       ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
+           hdr->b_l1hdr.b_state == arc_mfu);
+
+       DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
+       arc_access(hdr, hash_lock);
+       mutex_exit(hash_lock);
+
+       ARCSTAT_BUMP(arcstat_hits);
+       ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr),
+           demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data, metadata, hits);
  }
/* a generic arc_done_func_t which you can use */

Modified: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dbuf.c
==============================================================================
--- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dbuf.c  Mon Apr 16 
00:42:45 2018        (r332522)
+++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dbuf.c  Mon Apr 16 
00:54:58 2018        (r332523)
@@ -2579,8 +2579,10 @@ top:
                return (SET_ERROR(ENOENT));
        }
- if (db->db_buf != NULL)
+       if (db->db_buf != NULL) {
+               arc_buf_access(db->db_buf);
                ASSERT3P(db->db.db_data, ==, db->db_buf->b_data);
+       }
ASSERT(db->db_buf == NULL || arc_referenced(db->db_buf));
Modified: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/arc.h
==============================================================================
--- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/arc.h       Mon Apr 
16 00:42:45 2018        (r332522)
+++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/arc.h       Mon Apr 
16 00:54:58 2018        (r332523)
@@ -169,6 +169,7 @@ void arc_loan_inuse_buf(arc_buf_t *buf, void *tag);
  void arc_buf_destroy(arc_buf_t *buf, void *tag);
  int arc_buf_size(arc_buf_t *buf);
  int arc_buf_lsize(arc_buf_t *buf);
+void arc_buf_access(arc_buf_t *buf);
  void arc_release(arc_buf_t *buf, void *tag);
  int arc_released(arc_buf_t *buf);
  void arc_buf_freeze(arc_buf_t *buf);


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