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vfs: increment numvnodes without the vnode list lock unless under pressure

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The vnode list lock is only needed to reclaim free vnodes or kick the vnlru
thread (or to block and not miss a wake up (but note the sleep has a timeout so
this would not be a correctness issue)). Try to get away without the lock by
just doing an atomic increment.

The lock is contended e.g., during poudriere -j 104 where about half of all
acquires come from vnode allocation code.

Note the entire scheme needs a rewrite, the above just reduces it's SMP impact.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D23140
This commit is contained in:
Mateusz Guzik 2020-01-16 21:45:21 +00:00
parent b7f50b9ad1
commit 66f67d5e5e
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=356812
1 changed files with 23 additions and 6 deletions
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29
sys/kern/vfs_subr.c
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@ -1497,21 +1497,22 @@ vtryrecycle(struct vnode *vp)
* The routine can try to free a vnode or stall for up to 1 second waiting for
* vnlru to clear things up, but ultimately always performs a M_WAITOK allocation.
*/
static struct vnode *
vn_alloc(struct mount *mp)
static u_long vn_alloc_cyclecount;
static struct vnode * __noinline
vn_alloc_hard(struct mount *mp)
{
u_long rnumvnodes, rfreevnodes;
static u_long cyclecount;
mtx_lock(&vnode_list_mtx);
rnumvnodes = atomic_load_long(&numvnodes);
if (rnumvnodes + 1 < desiredvnodes) {
cyclecount = 0;
vn_alloc_cyclecount = 0;
goto alloc;
}
rfreevnodes = atomic_load_long(&freevnodes);
if (cyclecount++ >= rfreevnodes) {
cyclecount = 0;
if (vn_alloc_cyclecount++ >= rfreevnodes) {
vn_alloc_cyclecount = 0;
vstir = 1;
}
/*
@ -1546,6 +1547,22 @@ vn_alloc(struct mount *mp)
return (uma_zalloc(vnode_zone, M_WAITOK));
}
static struct vnode *
vn_alloc(struct mount *mp)
{
u_long rnumvnodes;
if (__predict_false(vn_alloc_cyclecount != 0))
return (vn_alloc_hard(mp));
rnumvnodes = atomic_fetchadd_long(&numvnodes, 1) + 1;
if (__predict_false(vnlru_under(rnumvnodes, vlowat))) {
atomic_subtract_long(&numvnodes, 1);
return (vn_alloc_hard(mp));
}
return (uma_zalloc(vnode_zone, M_WAITOK));
}
static void
vn_free(struct vnode *vp)
{