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Use atomics to modify numvnodes variable.

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This allows to mostly avoid lock usage in getnewvnode_[drop_]reserve(),
that reduces number of global vnode_free_list_mtx mutex acquisitions
from 4 to 2 per NFS request on ZFS, improving SMP scalability.

Reviewed by:	kib
MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.
This commit is contained in:
Alexander Motin 2014-06-08 15:38:40 +00:00
parent b563304c50
commit 590d636321
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=267232
1 changed files with 15 additions and 9 deletions
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24
sys/kern/vfs_subr.c
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@ -995,14 +995,25 @@ void
getnewvnode_reserve(u_int count)
{
struct thread *td;
long num;
td = curthread;
/* First try to be quick and racy. */
if (numvnodes + count <= desiredvnodes) {
num = atomic_fetchadd_long(&numvnodes, count);
if (num + count <= desiredvnodes) {
td->td_vp_reserv += count;
return;
} else
atomic_subtract_long(&numvnodes, count);
}
mtx_lock(&vnode_free_list_mtx);
while (count > 0) {
if (getnewvnode_wait(0) == 0) {
count--;
td->td_vp_reserv++;
numvnodes++;
atomic_add_long(&numvnodes, 1);
}
}
mtx_unlock(&vnode_free_list_mtx);
@ -1014,10 +1025,7 @@ getnewvnode_drop_reserve(void)
struct thread *td;
td = curthread;
mtx_lock(&vnode_free_list_mtx);
KASSERT(numvnodes >= td->td_vp_reserv, ("reserve too large"));
numvnodes -= td->td_vp_reserv;
mtx_unlock(&vnode_free_list_mtx);
atomic_subtract_long(&numvnodes, td->td_vp_reserv);
td->td_vp_reserv = 0;
}
@ -1054,7 +1062,7 @@ getnewvnode(const char *tag, struct mount *mp, struct vop_vector *vops,
return (error);
}
#endif
numvnodes++;
atomic_add_long(&numvnodes, 1);
mtx_unlock(&vnode_free_list_mtx);
alloc:
vp = (struct vnode *) uma_zalloc(vnode_zone, M_WAITOK|M_ZERO);
@ -2385,9 +2393,7 @@ vdropl(struct vnode *vp)
* The vnode has been marked for destruction, so free it.
*/
CTR2(KTR_VFS, "%s: destroying the vnode %p", __func__, vp);
mtx_lock(&vnode_free_list_mtx);
numvnodes--;
mtx_unlock(&vnode_free_list_mtx);
atomic_subtract_long(&numvnodes, 1);
bo = &vp->v_bufobj;
VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
("cleaned vnode still on the free list."));