From 85c43508f3cc5ff57b1cd53a42ea932c9c25464d Mon Sep 17 00:00:00 2001 From: Alexander Motin Date: Thu, 27 May 2021 16:27:29 -0400 Subject: [PATCH] Introduce write-mostly sums wmsum counters are a reduced version of aggsum counters, optimized for write-mostly scenarios. They do not provide optimized read functions, but instead allow much cheaper add function. The primary usage is infrequently read statistic counters, not requiring exact precision. The Linux implementation is directly mapped into percpu_counter KPI. The FreeBSD implementation is directly mapped into counter(9) KPI. In user-space due to lack of better implementation mapped to aggsum. Unfortunately neither Linux percpu_counter nor FreeBSD counter(9) provide sufficient functionality to completelly replace aggsum, so it still remains to be used for several hot counters. Reviewed-by: Paul Dagnelie Reviewed-by: Brian Behlendorf Reviewed-by: Ryan Moeller Signed-off-by: Alexander Motin Sponsored-By: iXsystems, Inc. Closes #12114 --- config/kernel-percpu.m4 | 27 +++++++++ include/os/freebsd/spl/sys/Makefile.am | 1 + include/os/freebsd/spl/sys/wmsum.h | 72 ++++++++++++++++++++++++ include/os/linux/spl/sys/Makefile.am | 1 + include/os/linux/spl/sys/wmsum.h | 76 +++++++++++++++++++++++++ include/sys/dataset_kstats.h | 20 +++---- lib/libspl/include/sys/Makefile.am | 1 + lib/libspl/include/sys/wmsum.h | 68 +++++++++++++++++++++++ module/zfs/arc.c | 77 +++++++++++++------------- module/zfs/dataset_kstats.c | 48 ++++++++-------- 10 files changed, 319 insertions(+), 72 deletions(-) create mode 100644 include/os/freebsd/spl/sys/wmsum.h create mode 100644 include/os/linux/spl/sys/wmsum.h create mode 100644 lib/libspl/include/sys/wmsum.h diff --git a/config/kernel-percpu.m4 b/config/kernel-percpu.m4 index 700d97a258..5125dd5c5b 100644 --- a/config/kernel-percpu.m4 +++ b/config/kernel-percpu.m4 @@ -25,6 +25,31 @@ AC_DEFUN([ZFS_AC_KERNEL_PERCPU_COUNTER_INIT], [ ]) ]) +dnl # +dnl # 4.13 API change, +dnl # __percpu_counter_add() was renamed to percpu_counter_add_batch(). +dnl # +AC_DEFUN([ZFS_AC_KERNEL_SRC_PERCPU_COUNTER_ADD_BATCH], [ + ZFS_LINUX_TEST_SRC([percpu_counter_add_batch], [ + #include + ],[ + struct percpu_counter counter; + + percpu_counter_add_batch(&counter, 1, 1); + ]) +]) + +AC_DEFUN([ZFS_AC_KERNEL_PERCPU_COUNTER_ADD_BATCH], [ + AC_MSG_CHECKING([whether percpu_counter_add_batch() is defined]) + ZFS_LINUX_TEST_RESULT([percpu_counter_add_batch], [ + AC_MSG_RESULT(yes) + AC_DEFINE(HAVE_PERCPU_COUNTER_ADD_BATCH, 1, + [percpu_counter_add_batch() is defined]) + ],[ + AC_MSG_RESULT(no) + ]) +]) + dnl # dnl # 5.10 API change, dnl # The "count" was moved into ref->data, from ref @@ -51,10 +76,12 @@ AC_DEFUN([ZFS_AC_KERNEL_PERCPU_REF_COUNT_IN_DATA], [ ]) AC_DEFUN([ZFS_AC_KERNEL_SRC_PERCPU], [ ZFS_AC_KERNEL_SRC_PERCPU_COUNTER_INIT + ZFS_AC_KERNEL_SRC_PERCPU_COUNTER_ADD_BATCH ZFS_AC_KERNEL_SRC_PERCPU_REF_COUNT_IN_DATA ]) AC_DEFUN([ZFS_AC_KERNEL_PERCPU], [ ZFS_AC_KERNEL_PERCPU_COUNTER_INIT + ZFS_AC_KERNEL_PERCPU_COUNTER_ADD_BATCH ZFS_AC_KERNEL_PERCPU_REF_COUNT_IN_DATA ]) diff --git a/include/os/freebsd/spl/sys/Makefile.am b/include/os/freebsd/spl/sys/Makefile.am index 4bb9beffc7..232aaf569f 100644 --- a/include/os/freebsd/spl/sys/Makefile.am +++ b/include/os/freebsd/spl/sys/Makefile.am @@ -68,6 +68,7 @@ KERNEL_H = \ vmsystm.h \ vnode_impl.h \ vnode.h \ + wmsum.h \ zmod.h \ zone.h diff --git a/include/os/freebsd/spl/sys/wmsum.h b/include/os/freebsd/spl/sys/wmsum.h new file mode 100644 index 0000000000..9fdd1901b7 --- /dev/null +++ b/include/os/freebsd/spl/sys/wmsum.h @@ -0,0 +1,72 @@ +/* + * CDDL HEADER START + * + * This file and its contents are supplied under the terms of the + * Common Development and Distribution License ("CDDL"), version 1.0. + * You may only use this file in accordance with the terms of version + * 1.0 of the CDDL. + * + * A full copy of the text of the CDDL should have accompanied this + * source. A copy of the CDDL is also available via the Internet at + * http://www.illumos.org/license/CDDL. + * + * CDDL HEADER END + */ + +/* + * wmsum counters are a reduced version of aggsum counters, optimized for + * write-mostly scenarios. They do not provide optimized read functions, + * but instead allow much cheaper add function. The primary usage is + * infrequently read statistic counters, not requiring exact precision. + * + * The FreeBSD implementation is directly mapped into counter(9) KPI. + */ + +#ifndef _SYS_WMSUM_H +#define _SYS_WMSUM_H + +#include +#include +#include +#include + +#ifdef __cplusplus +extern "C" { +#endif + +#define wmsum_t counter_u64_t + +static inline void +wmsum_init(wmsum_t *ws, uint64_t value) +{ + + *ws = counter_u64_alloc(M_WAITOK); + counter_u64_add(*ws, value); +} + +static inline void +wmsum_fini(wmsum_t *ws) +{ + + counter_u64_free(*ws); +} + +static inline uint64_t +wmsum_value(wmsum_t *ws) +{ + + return (counter_u64_fetch(*ws)); +} + +static inline void +wmsum_add(wmsum_t *ws, int64_t delta) +{ + + counter_u64_add(*ws, delta); +} + +#ifdef __cplusplus +} +#endif + +#endif /* _SYS_WMSUM_H */ diff --git a/include/os/linux/spl/sys/Makefile.am b/include/os/linux/spl/sys/Makefile.am index a5b0d78e8b..48c27f970f 100644 --- a/include/os/linux/spl/sys/Makefile.am +++ b/include/os/linux/spl/sys/Makefile.am @@ -54,6 +54,7 @@ KERNEL_H = \ vmsystm.h \ vnode.h \ wait.h \ + wmsum.h \ zmod.h \ zone.h diff --git a/include/os/linux/spl/sys/wmsum.h b/include/os/linux/spl/sys/wmsum.h new file mode 100644 index 0000000000..0871bd6950 --- /dev/null +++ b/include/os/linux/spl/sys/wmsum.h @@ -0,0 +1,76 @@ +/* + * CDDL HEADER START + * + * This file and its contents are supplied under the terms of the + * Common Development and Distribution License ("CDDL"), version 1.0. + * You may only use this file in accordance with the terms of version + * 1.0 of the CDDL. + * + * A full copy of the text of the CDDL should have accompanied this + * source. A copy of the CDDL is also available via the Internet at + * http://www.illumos.org/license/CDDL. + * + * CDDL HEADER END + */ + +/* + * wmsum counters are a reduced version of aggsum counters, optimized for + * write-mostly scenarios. They do not provide optimized read functions, + * but instead allow much cheaper add function. The primary usage is + * infrequently read statistic counters, not requiring exact precision. + * + * The Linux implementation is directly mapped into percpu_counter KPI. + */ + +#ifndef _SYS_WMSUM_H +#define _SYS_WMSUM_H + +#include + +#ifdef __cplusplus +extern "C" { +#endif + +typedef struct percpu_counter wmsum_t; + +static inline void +wmsum_init(wmsum_t *ws, uint64_t value) +{ + +#ifdef HAVE_PERCPU_COUNTER_INIT_WITH_GFP + percpu_counter_init(ws, value, GFP_KERNEL); +#else + percpu_counter_init(ws, value); +#endif +} + +static inline void +wmsum_fini(wmsum_t *ws) +{ + + percpu_counter_destroy(ws); +} + +static inline uint64_t +wmsum_value(wmsum_t *ws) +{ + + return (percpu_counter_sum(ws)); +} + +static inline void +wmsum_add(wmsum_t *ws, int64_t delta) +{ + +#ifdef HAVE_PERCPU_COUNTER_ADD_BATCH + percpu_counter_add_batch(ws, delta, INT_MAX / 2); +#else + __percpu_counter_add(ws, delta, INT_MAX / 2); +#endif +} + +#ifdef __cplusplus +} +#endif + +#endif /* _SYS_WMSUM_H */ diff --git a/include/sys/dataset_kstats.h b/include/sys/dataset_kstats.h index 667d1b85fa..b165b98576 100644 --- a/include/sys/dataset_kstats.h +++ b/include/sys/dataset_kstats.h @@ -27,18 +27,18 @@ #ifndef _SYS_DATASET_KSTATS_H #define _SYS_DATASET_KSTATS_H -#include +#include #include #include -typedef struct dataset_aggsum_stats_t { - aggsum_t das_writes; - aggsum_t das_nwritten; - aggsum_t das_reads; - aggsum_t das_nread; - aggsum_t das_nunlinks; - aggsum_t das_nunlinked; -} dataset_aggsum_stats_t; +typedef struct dataset_sum_stats_t { + wmsum_t dss_writes; + wmsum_t dss_nwritten; + wmsum_t dss_reads; + wmsum_t dss_nread; + wmsum_t dss_nunlinks; + wmsum_t dss_nunlinked; +} dataset_sum_stats_t; typedef struct dataset_kstat_values { kstat_named_t dkv_ds_name; @@ -59,7 +59,7 @@ typedef struct dataset_kstat_values { } dataset_kstat_values_t; typedef struct dataset_kstats { - dataset_aggsum_stats_t dk_aggsums; + dataset_sum_stats_t dk_sums; kstat_t *dk_kstats; } dataset_kstats_t; diff --git a/lib/libspl/include/sys/Makefile.am b/lib/libspl/include/sys/Makefile.am index 53a36f6bfb..6816a01253 100644 --- a/lib/libspl/include/sys/Makefile.am +++ b/lib/libspl/include/sys/Makefile.am @@ -44,4 +44,5 @@ libspl_HEADERS = \ varargs.h \ vnode.h \ vtoc.h \ + wmsum.h \ zone.h diff --git a/lib/libspl/include/sys/wmsum.h b/lib/libspl/include/sys/wmsum.h new file mode 100644 index 0000000000..0679af73ce --- /dev/null +++ b/lib/libspl/include/sys/wmsum.h @@ -0,0 +1,68 @@ +/* + * CDDL HEADER START + * + * This file and its contents are supplied under the terms of the + * Common Development and Distribution License ("CDDL"), version 1.0. + * You may only use this file in accordance with the terms of version + * 1.0 of the CDDL. + * + * A full copy of the text of the CDDL should have accompanied this + * source. A copy of the CDDL is also available via the Internet at + * http://www.illumos.org/license/CDDL. + * + * CDDL HEADER END + */ + +/* + * wmsum counters are a reduced version of aggsum counters, optimized for + * write-mostly scenarios. They do not provide optimized read functions, + * but instead allow much cheaper add function. The primary usage is + * infrequently read statistic counters, not requiring exact precision. + * + * In user-space due to lack of better implementation mapped to aggsum. + */ + +#ifndef _SYS_WMSUM_H +#define _SYS_WMSUM_H + +#include + +#ifdef __cplusplus +extern "C" { +#endif + +#define wmsum_t aggsum_t + +static inline void +wmsum_init(wmsum_t *ws, uint64_t value) +{ + + aggsum_init(ws, value); +} + +static inline void +wmsum_fini(wmsum_t *ws) +{ + + aggsum_fini(ws); +} + +static inline uint64_t +wmsum_value(wmsum_t *ws) +{ + + return (aggsum_value(ws)); +} + +static inline void +wmsum_add(wmsum_t *ws, int64_t delta) +{ + + aggsum_add(ws, delta); +} + +#ifdef __cplusplus +} +#endif + +#endif /* _SYS_WMSUM_H */ diff --git a/module/zfs/arc.c b/module/zfs/arc.c index f0ae3938a3..716f108ebd 100644 --- a/module/zfs/arc.c +++ b/module/zfs/arc.c @@ -305,6 +305,7 @@ #include #include #include +#include #include #include #include @@ -692,14 +693,14 @@ arc_state_t *arc_mfu; */ aggsum_t arc_size; aggsum_t arc_meta_used; -aggsum_t astat_data_size; -aggsum_t astat_metadata_size; -aggsum_t astat_dbuf_size; +wmsum_t astat_data_size; +wmsum_t astat_metadata_size; +wmsum_t astat_dbuf_size; aggsum_t astat_dnode_size; -aggsum_t astat_bonus_size; -aggsum_t astat_hdr_size; +wmsum_t astat_bonus_size; +wmsum_t astat_hdr_size; aggsum_t astat_l2_hdr_size; -aggsum_t astat_abd_chunk_waste_size; +wmsum_t astat_abd_chunk_waste_size; hrtime_t arc_growtime; list_t arc_prune_list; @@ -2645,22 +2646,22 @@ arc_space_consume(uint64_t space, arc_space_type_t type) default: break; case ARC_SPACE_DATA: - aggsum_add(&astat_data_size, space); + wmsum_add(&astat_data_size, space); break; case ARC_SPACE_META: - aggsum_add(&astat_metadata_size, space); + wmsum_add(&astat_metadata_size, space); break; case ARC_SPACE_BONUS: - aggsum_add(&astat_bonus_size, space); + wmsum_add(&astat_bonus_size, space); break; case ARC_SPACE_DNODE: aggsum_add(&astat_dnode_size, space); break; case ARC_SPACE_DBUF: - aggsum_add(&astat_dbuf_size, space); + wmsum_add(&astat_dbuf_size, space); break; case ARC_SPACE_HDRS: - aggsum_add(&astat_hdr_size, space); + wmsum_add(&astat_hdr_size, space); break; case ARC_SPACE_L2HDRS: aggsum_add(&astat_l2_hdr_size, space); @@ -2672,7 +2673,7 @@ arc_space_consume(uint64_t space, arc_space_type_t type) * scatter ABD's come from the ARC, because other users are * very short-lived. */ - aggsum_add(&astat_abd_chunk_waste_size, space); + wmsum_add(&astat_abd_chunk_waste_size, space); break; } @@ -2691,28 +2692,28 @@ arc_space_return(uint64_t space, arc_space_type_t type) default: break; case ARC_SPACE_DATA: - aggsum_add(&astat_data_size, -space); + wmsum_add(&astat_data_size, -space); break; case ARC_SPACE_META: - aggsum_add(&astat_metadata_size, -space); + wmsum_add(&astat_metadata_size, -space); break; case ARC_SPACE_BONUS: - aggsum_add(&astat_bonus_size, -space); + wmsum_add(&astat_bonus_size, -space); break; case ARC_SPACE_DNODE: aggsum_add(&astat_dnode_size, -space); break; case ARC_SPACE_DBUF: - aggsum_add(&astat_dbuf_size, -space); + wmsum_add(&astat_dbuf_size, -space); break; case ARC_SPACE_HDRS: - aggsum_add(&astat_hdr_size, -space); + wmsum_add(&astat_hdr_size, -space); break; case ARC_SPACE_L2HDRS: aggsum_add(&astat_l2_hdr_size, -space); break; case ARC_SPACE_ABD_CHUNK_WASTE: - aggsum_add(&astat_abd_chunk_waste_size, -space); + wmsum_add(&astat_abd_chunk_waste_size, -space); break; } @@ -7275,21 +7276,21 @@ arc_kstat_update(kstat_t *ksp, int rw) ARCSTAT(arcstat_size) = aggsum_value(&arc_size); ARCSTAT(arcstat_meta_used) = aggsum_value(&arc_meta_used); - ARCSTAT(arcstat_data_size) = aggsum_value(&astat_data_size); + ARCSTAT(arcstat_data_size) = wmsum_value(&astat_data_size); ARCSTAT(arcstat_metadata_size) = - aggsum_value(&astat_metadata_size); - ARCSTAT(arcstat_hdr_size) = aggsum_value(&astat_hdr_size); + wmsum_value(&astat_metadata_size); + ARCSTAT(arcstat_hdr_size) = wmsum_value(&astat_hdr_size); ARCSTAT(arcstat_l2_hdr_size) = aggsum_value(&astat_l2_hdr_size); - ARCSTAT(arcstat_dbuf_size) = aggsum_value(&astat_dbuf_size); + ARCSTAT(arcstat_dbuf_size) = wmsum_value(&astat_dbuf_size); #if defined(COMPAT_FREEBSD11) - ARCSTAT(arcstat_other_size) = aggsum_value(&astat_bonus_size) + + ARCSTAT(arcstat_other_size) = wmsum_value(&astat_bonus_size) + aggsum_value(&astat_dnode_size) + - aggsum_value(&astat_dbuf_size); + wmsum_value(&astat_dbuf_size); #endif ARCSTAT(arcstat_dnode_size) = aggsum_value(&astat_dnode_size); - ARCSTAT(arcstat_bonus_size) = aggsum_value(&astat_bonus_size); + ARCSTAT(arcstat_bonus_size) = wmsum_value(&astat_bonus_size); ARCSTAT(arcstat_abd_chunk_waste_size) = - aggsum_value(&astat_abd_chunk_waste_size); + wmsum_value(&astat_abd_chunk_waste_size); as->arcstat_memory_all_bytes.value.ui64 = arc_all_memory(); @@ -7522,14 +7523,14 @@ arc_state_init(void) aggsum_init(&arc_meta_used, 0); aggsum_init(&arc_size, 0); - aggsum_init(&astat_data_size, 0); - aggsum_init(&astat_metadata_size, 0); - aggsum_init(&astat_hdr_size, 0); + wmsum_init(&astat_data_size, 0); + wmsum_init(&astat_metadata_size, 0); + wmsum_init(&astat_hdr_size, 0); aggsum_init(&astat_l2_hdr_size, 0); - aggsum_init(&astat_bonus_size, 0); + wmsum_init(&astat_bonus_size, 0); aggsum_init(&astat_dnode_size, 0); - aggsum_init(&astat_dbuf_size, 0); - aggsum_init(&astat_abd_chunk_waste_size, 0); + wmsum_init(&astat_dbuf_size, 0); + wmsum_init(&astat_abd_chunk_waste_size, 0); arc_anon->arcs_state = ARC_STATE_ANON; arc_mru->arcs_state = ARC_STATE_MRU; @@ -7575,14 +7576,14 @@ arc_state_fini(void) aggsum_fini(&arc_meta_used); aggsum_fini(&arc_size); - aggsum_fini(&astat_data_size); - aggsum_fini(&astat_metadata_size); - aggsum_fini(&astat_hdr_size); + wmsum_fini(&astat_data_size); + wmsum_fini(&astat_metadata_size); + wmsum_fini(&astat_hdr_size); aggsum_fini(&astat_l2_hdr_size); - aggsum_fini(&astat_bonus_size); + wmsum_fini(&astat_bonus_size); aggsum_fini(&astat_dnode_size); - aggsum_fini(&astat_dbuf_size); - aggsum_fini(&astat_abd_chunk_waste_size); + wmsum_fini(&astat_dbuf_size); + wmsum_fini(&astat_abd_chunk_waste_size); } uint64_t diff --git a/module/zfs/dataset_kstats.c b/module/zfs/dataset_kstats.c index e46a0926d5..3fbb24ddef 100644 --- a/module/zfs/dataset_kstats.c +++ b/module/zfs/dataset_kstats.c @@ -50,17 +50,17 @@ dataset_kstats_update(kstat_t *ksp, int rw) dataset_kstat_values_t *dkv = dk->dk_kstats->ks_data; dkv->dkv_writes.value.ui64 = - aggsum_value(&dk->dk_aggsums.das_writes); + wmsum_value(&dk->dk_sums.dss_writes); dkv->dkv_nwritten.value.ui64 = - aggsum_value(&dk->dk_aggsums.das_nwritten); + wmsum_value(&dk->dk_sums.dss_nwritten); dkv->dkv_reads.value.ui64 = - aggsum_value(&dk->dk_aggsums.das_reads); + wmsum_value(&dk->dk_sums.dss_reads); dkv->dkv_nread.value.ui64 = - aggsum_value(&dk->dk_aggsums.das_nread); + wmsum_value(&dk->dk_sums.dss_nread); dkv->dkv_nunlinks.value.ui64 = - aggsum_value(&dk->dk_aggsums.das_nunlinks); + wmsum_value(&dk->dk_sums.dss_nunlinks); dkv->dkv_nunlinked.value.ui64 = - aggsum_value(&dk->dk_aggsums.das_nunlinked); + wmsum_value(&dk->dk_sums.dss_nunlinked); return (0); } @@ -140,12 +140,12 @@ dataset_kstats_create(dataset_kstats_t *dk, objset_t *objset) kstat_install(kstat); dk->dk_kstats = kstat; - aggsum_init(&dk->dk_aggsums.das_writes, 0); - aggsum_init(&dk->dk_aggsums.das_nwritten, 0); - aggsum_init(&dk->dk_aggsums.das_reads, 0); - aggsum_init(&dk->dk_aggsums.das_nread, 0); - aggsum_init(&dk->dk_aggsums.das_nunlinks, 0); - aggsum_init(&dk->dk_aggsums.das_nunlinked, 0); + wmsum_init(&dk->dk_sums.dss_writes, 0); + wmsum_init(&dk->dk_sums.dss_nwritten, 0); + wmsum_init(&dk->dk_sums.dss_reads, 0); + wmsum_init(&dk->dk_sums.dss_nread, 0); + wmsum_init(&dk->dk_sums.dss_nunlinks, 0); + wmsum_init(&dk->dk_sums.dss_nunlinked, 0); } void @@ -162,12 +162,12 @@ dataset_kstats_destroy(dataset_kstats_t *dk) kstat_delete(dk->dk_kstats); dk->dk_kstats = NULL; - aggsum_fini(&dk->dk_aggsums.das_writes); - aggsum_fini(&dk->dk_aggsums.das_nwritten); - aggsum_fini(&dk->dk_aggsums.das_reads); - aggsum_fini(&dk->dk_aggsums.das_nread); - aggsum_fini(&dk->dk_aggsums.das_nunlinks); - aggsum_fini(&dk->dk_aggsums.das_nunlinked); + wmsum_fini(&dk->dk_sums.dss_writes); + wmsum_fini(&dk->dk_sums.dss_nwritten); + wmsum_fini(&dk->dk_sums.dss_reads); + wmsum_fini(&dk->dk_sums.dss_nread); + wmsum_fini(&dk->dk_sums.dss_nunlinks); + wmsum_fini(&dk->dk_sums.dss_nunlinked); } void @@ -179,8 +179,8 @@ dataset_kstats_update_write_kstats(dataset_kstats_t *dk, if (dk->dk_kstats == NULL) return; - aggsum_add(&dk->dk_aggsums.das_writes, 1); - aggsum_add(&dk->dk_aggsums.das_nwritten, nwritten); + wmsum_add(&dk->dk_sums.dss_writes, 1); + wmsum_add(&dk->dk_sums.dss_nwritten, nwritten); } void @@ -192,8 +192,8 @@ dataset_kstats_update_read_kstats(dataset_kstats_t *dk, if (dk->dk_kstats == NULL) return; - aggsum_add(&dk->dk_aggsums.das_reads, 1); - aggsum_add(&dk->dk_aggsums.das_nread, nread); + wmsum_add(&dk->dk_sums.dss_reads, 1); + wmsum_add(&dk->dk_sums.dss_nread, nread); } void @@ -202,7 +202,7 @@ dataset_kstats_update_nunlinks_kstat(dataset_kstats_t *dk, int64_t delta) if (dk->dk_kstats == NULL) return; - aggsum_add(&dk->dk_aggsums.das_nunlinks, delta); + wmsum_add(&dk->dk_sums.dss_nunlinks, delta); } void @@ -211,5 +211,5 @@ dataset_kstats_update_nunlinked_kstat(dataset_kstats_t *dk, int64_t delta) if (dk->dk_kstats == NULL) return; - aggsum_add(&dk->dk_aggsums.das_nunlinked, delta); + wmsum_add(&dk->dk_sums.dss_nunlinked, delta); }