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ARC: Remove b_cv from struct l1arc_buf_hdr
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Earlier as part of #14123 I've removed one use of b_cv.  This patch
reuses the same approach to remove the other one from much more
rare code path.

This saves 16 bytes of L1 ARC header on FreeBSD (reducing it from
200 to 184 bytes) and seems even more on Linux.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #15340
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amotin authored Oct 4, 2023
1 parent f795e90 commit 2a6c621
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Showing 2 changed files with 22 additions and 14 deletions.
2 changes: 0 additions & 2 deletions include/sys/arc_impl.h
Original file line number Diff line number Diff line change
Expand Up @@ -159,8 +159,6 @@ struct arc_write_callback {
* these two allocation states.
*/
typedef struct l1arc_buf_hdr {
/* for waiting on reads to complete */
kcondvar_t b_cv;
uint8_t b_byteswap;

/* protected by arc state mutex */
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34 changes: 22 additions & 12 deletions module/zfs/arc.c
Original file line number Diff line number Diff line change
Expand Up @@ -1151,7 +1151,6 @@ hdr_full_cons(void *vbuf, void *unused, int kmflag)

memset(hdr, 0, HDR_FULL_SIZE);
hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
cv_init(&hdr->b_l1hdr.b_cv, NULL, CV_DEFAULT, NULL);
zfs_refcount_create(&hdr->b_l1hdr.b_refcnt);
#ifdef ZFS_DEBUG
mutex_init(&hdr->b_l1hdr.b_freeze_lock, NULL, MUTEX_DEFAULT, NULL);
Expand Down Expand Up @@ -1211,7 +1210,6 @@ hdr_full_dest(void *vbuf, void *unused)
arc_buf_hdr_t *hdr = vbuf;

ASSERT(HDR_EMPTY(hdr));
cv_destroy(&hdr->b_l1hdr.b_cv);
zfs_refcount_destroy(&hdr->b_l1hdr.b_refcnt);
#ifdef ZFS_DEBUG
mutex_destroy(&hdr->b_l1hdr.b_freeze_lock);
Expand Down Expand Up @@ -5586,13 +5584,6 @@ arc_read_done(zio_t *zio)
buf_hash_remove(hdr);
}

/*
* Broadcast before we drop the hash_lock to avoid the possibility
* that the hdr (and hence the cv) might be freed before we get to
* the cv_broadcast().
*/
cv_broadcast(&hdr->b_l1hdr.b_cv);

arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
(void) remove_reference(hdr, hdr);

Expand Down Expand Up @@ -5787,8 +5778,7 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
}
acb->acb_zio_head = head_zio;
acb->acb_next = hdr->b_l1hdr.b_acb;
if (hdr->b_l1hdr.b_acb)
hdr->b_l1hdr.b_acb->acb_prev = acb;
hdr->b_l1hdr.b_acb->acb_prev = acb;
hdr->b_l1hdr.b_acb = acb;
}
mutex_exit(hash_lock);
Expand Down Expand Up @@ -5928,8 +5918,28 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
* and so the performance impact shouldn't
* matter.
*/
cv_wait(&hdr->b_l1hdr.b_cv, hash_lock);
arc_callback_t *acb = kmem_zalloc(
sizeof (arc_callback_t), KM_SLEEP);
acb->acb_wait = B_TRUE;
mutex_init(&acb->acb_wait_lock, NULL,
MUTEX_DEFAULT, NULL);
cv_init(&acb->acb_wait_cv, NULL, CV_DEFAULT,
NULL);
acb->acb_zio_head =
hdr->b_l1hdr.b_acb->acb_zio_head;
acb->acb_next = hdr->b_l1hdr.b_acb;
hdr->b_l1hdr.b_acb->acb_prev = acb;
hdr->b_l1hdr.b_acb = acb;
mutex_exit(hash_lock);
mutex_enter(&acb->acb_wait_lock);
while (acb->acb_wait) {
cv_wait(&acb->acb_wait_cv,
&acb->acb_wait_lock);
}
mutex_exit(&acb->acb_wait_lock);
mutex_destroy(&acb->acb_wait_lock);
cv_destroy(&acb->acb_wait_cv);
kmem_free(acb, sizeof (arc_callback_t));
goto top;
}
}
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