git: fdb1dbb1cc06 - main - vm: read-locked fault handling for backing objects
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Date: Sat, 11 Mar 2023 11:08:43 UTC
The branch main has been updated by mjg:
URL: https://cgit.FreeBSD.org/src/commit/?id=fdb1dbb1cc0608edd54451050fa56b84a303c8a6
commit fdb1dbb1cc0608edd54451050fa56b84a303c8a6
Author: Mateusz Guzik <mjg@FreeBSD.org>
AuthorDate: 2023-03-07 20:56:54 +0000
Commit: Mateusz Guzik <mjg@FreeBSD.org>
CommitDate: 2023-03-11 11:08:21 +0000
vm: read-locked fault handling for backing objects
This is almost the simplest patch which manages to avoid write locking
for backing objects, as a result mostly fixing vm object contention
problems.
What is not fixed:
1. cacheline ping pong due to read-locks
2. cacheline ping pong due to pip
3. cacheling ping pong due to object busying
4. write locking on first object
On top of it the use of VM_OBJECT_UNLOCK instead of explicitly tracking
the state is slower multithreaded that it needs to be, done for
simplicity for the time being.
Sample lock profiling results doing -j 104 buildkernel on tmpfs:
before:
71446200 (rw:vmobject)
14689706 (sx:vm map (user))
4166251 (rw:pmap pv list)
2799924 (spin mutex:turnstile chain)
after:
19940411 (rw:vmobject)
8166012 (rw:pmap pv list)
6017608 (sx:vm map (user))
1151416 (sleep mutex:pipe mutex)
Reviewed by: kib
Reviewed by: markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D38964
---
sys/vm/vm_fault.c | 81 +++++++++++++++++++++++++++++++++++++++++++++----------
1 file changed, 67 insertions(+), 14 deletions(-)
diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index 2afe5a19d2d7..5df667052615 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -132,6 +132,7 @@ struct faultstate {
struct timeval oom_start_time;
bool oom_started;
int nera;
+ bool can_read_lock;
/* Page reference for cow. */
vm_page_t m_cow;
@@ -170,6 +171,12 @@ enum fault_status {
FAULT_PROTECTION_FAILURE, /* Invalid access. */
};
+enum fault_next_status {
+ FAULT_NEXT_GOTOBJ = 1,
+ FAULT_NEXT_NOOBJ,
+ FAULT_NEXT_RESTART,
+};
+
static void vm_fault_dontneed(const struct faultstate *fs, vm_offset_t vaddr,
int ahead);
static void vm_fault_prefault(const struct faultstate *fs, vm_offset_t addra,
@@ -278,7 +285,7 @@ static void
unlock_and_deallocate(struct faultstate *fs)
{
- VM_OBJECT_WUNLOCK(fs->object);
+ VM_OBJECT_UNLOCK(fs->object);
fault_deallocate(fs);
}
@@ -736,6 +743,26 @@ vm_fault_trap(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
return (result);
}
+static bool
+vm_fault_object_ensure_wlocked(struct faultstate *fs)
+{
+ if (fs->object == fs->first_object)
+ VM_OBJECT_ASSERT_WLOCKED(fs->object);
+
+ if (!fs->can_read_lock) {
+ VM_OBJECT_ASSERT_WLOCKED(fs->object);
+ return (true);
+ }
+
+ if (VM_OBJECT_WOWNED(fs->object))
+ return (true);
+
+ if (VM_OBJECT_TRYUPGRADE(fs->object))
+ return (true);
+
+ return (false);
+}
+
static enum fault_status
vm_fault_lock_vnode(struct faultstate *fs, bool objlocked)
{
@@ -1042,12 +1069,15 @@ vm_fault_cow(struct faultstate *fs)
curthread->td_cow++;
}
-static bool
+static enum fault_next_status
vm_fault_next(struct faultstate *fs)
{
vm_object_t next_object;
- VM_OBJECT_ASSERT_WLOCKED(fs->object);
+ if (fs->object == fs->first_object || !fs->can_read_lock)
+ VM_OBJECT_ASSERT_WLOCKED(fs->object);
+ else
+ VM_OBJECT_ASSERT_LOCKED(fs->object);
/*
* The requested page does not exist at this object/
@@ -1062,8 +1092,14 @@ vm_fault_next(struct faultstate *fs)
if (fs->object == fs->first_object) {
fs->first_m = fs->m;
fs->m = NULL;
- } else
+ } else {
+ if (!vm_fault_object_ensure_wlocked(fs)) {
+ fs->can_read_lock = false;
+ unlock_and_deallocate(fs);
+ return (FAULT_NEXT_RESTART);
+ }
fault_page_free(&fs->m);
+ }
/*
* Move on to the next object. Lock the next object before
@@ -1071,18 +1107,21 @@ vm_fault_next(struct faultstate *fs)
*/
next_object = fs->object->backing_object;
if (next_object == NULL)
- return (false);
+ return (FAULT_NEXT_NOOBJ);
MPASS(fs->first_m != NULL);
KASSERT(fs->object != next_object, ("object loop %p", next_object));
- VM_OBJECT_WLOCK(next_object);
+ if (fs->can_read_lock)
+ VM_OBJECT_RLOCK(next_object);
+ else
+ VM_OBJECT_WLOCK(next_object);
vm_object_pip_add(next_object, 1);
if (fs->object != fs->first_object)
vm_object_pip_wakeup(fs->object);
fs->pindex += OFF_TO_IDX(fs->object->backing_object_offset);
- VM_OBJECT_WUNLOCK(fs->object);
+ VM_OBJECT_UNLOCK(fs->object);
fs->object = next_object;
- return (true);
+ return (FAULT_NEXT_GOTOBJ);
}
static void
@@ -1364,7 +1403,7 @@ vm_fault_busy_sleep(struct faultstate *fs)
unlock_map(fs);
if (fs->m != vm_page_lookup(fs->object, fs->pindex) ||
!vm_page_busy_sleep(fs->m, "vmpfw", 0))
- VM_OBJECT_WUNLOCK(fs->object);
+ VM_OBJECT_UNLOCK(fs->object);
VM_CNT_INC(v_intrans);
vm_object_deallocate(fs->first_object);
}
@@ -1383,7 +1422,10 @@ vm_fault_object(struct faultstate *fs, int *behindp, int *aheadp)
enum fault_status res;
bool dead;
- VM_OBJECT_ASSERT_WLOCKED(fs->object);
+ if (fs->object == fs->first_object || !fs->can_read_lock)
+ VM_OBJECT_ASSERT_WLOCKED(fs->object);
+ else
+ VM_OBJECT_ASSERT_LOCKED(fs->object);
/*
* If the object is marked for imminent termination, we retry
@@ -1415,7 +1457,7 @@ vm_fault_object(struct faultstate *fs, int *behindp, int *aheadp)
* done.
*/
if (vm_page_all_valid(fs->m)) {
- VM_OBJECT_WUNLOCK(fs->object);
+ VM_OBJECT_UNLOCK(fs->object);
return (FAULT_SOFT);
}
}
@@ -1427,6 +1469,11 @@ vm_fault_object(struct faultstate *fs, int *behindp, int *aheadp)
*/
if (fs->m == NULL && (fault_object_needs_getpages(fs->object) ||
fs->object == fs->first_object)) {
+ if (!vm_fault_object_ensure_wlocked(fs)) {
+ fs->can_read_lock = false;
+ unlock_and_deallocate(fs);
+ return (FAULT_RESTART);
+ }
res = vm_fault_allocate(fs);
if (res != FAULT_CONTINUE)
return (res);
@@ -1448,7 +1495,7 @@ vm_fault_object(struct faultstate *fs, int *behindp, int *aheadp)
* prevents simultaneous faults and collapses while
* the object lock is dropped.
*/
- VM_OBJECT_WUNLOCK(fs->object);
+ VM_OBJECT_UNLOCK(fs->object);
res = vm_fault_getpages(fs, behindp, aheadp);
if (res == FAULT_CONTINUE)
VM_OBJECT_WLOCK(fs->object);
@@ -1465,6 +1512,7 @@ vm_fault(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
struct faultstate fs;
int ahead, behind, faultcount, rv;
enum fault_status res;
+ enum fault_next_status res_next;
bool hardfault;
VM_CNT_INC(v_vm_faults);
@@ -1480,6 +1528,7 @@ vm_fault(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
fs.lookup_still_valid = false;
fs.oom_started = false;
fs.nera = -1;
+ fs.can_read_lock = true;
faultcount = 0;
hardfault = false;
@@ -1590,15 +1639,19 @@ RetryFault:
* traverse into a backing object or zero fill if none is
* found.
*/
- if (vm_fault_next(&fs))
+ res_next = vm_fault_next(&fs);
+ if (res_next == FAULT_NEXT_RESTART)
+ goto RetryFault;
+ else if (res_next == FAULT_NEXT_GOTOBJ)
continue;
+ MPASS(res_next == FAULT_NEXT_NOOBJ);
if ((fs.fault_flags & VM_FAULT_NOFILL) != 0) {
if (fs.first_object == fs.object)
fault_page_free(&fs.first_m);
unlock_and_deallocate(&fs);
return (KERN_OUT_OF_BOUNDS);
}
- VM_OBJECT_WUNLOCK(fs.object);
+ VM_OBJECT_UNLOCK(fs.object);
vm_fault_zerofill(&fs);
/* Don't try to prefault neighboring pages. */
faultcount = 1;