Re: [PATCH] pv-qspinlock: Try to re-hash the lock after spurious_wakeup

From: xinhui
Date: Fri May 27 2016 - 06:33:35 EST

On 2016å05æ27æ 02:31, Waiman Long wrote:
On 05/25/2016 02:09 AM, Pan Xinhui wrote:
In pv_wait_head_or_lock, if there is a spurious_wakeup, and it fails to
get the lock as there is lock stealing, then after a short spin, we need
hash the lock again and enter pv_wait to yield.

Currently after a spurious_wakeup, as l->locked is not _Q_SLOW_VAL,
pv_wait might do nothing and return directly, that is not
paravirt-friendly because pv_wait_head_or_lock will just spin on the
lock then.

Signed-off-by: Pan Xinhui<xinhui.pan@xxxxxxxxxxxxxxxxxx>
kernel/locking/qspinlock_paravirt.h | 39 +++++++++++++++++++++++++++++--------
1 file changed, 31 insertions(+), 8 deletions(-)

Is this a problem you can easily reproduce on PPC? I have not observed this issue when testing on x86.

Hi, Waiman
I notice the spurious_wakeup count is very high when I do benchmark tests and stress tests. So after a simple investigation,
I find pv_wait_head_or_lock() just keep loops.

Here is my story, in my pv-qspinlcok patchset V1&&v2, pv_wait on ppc ignore the first two parameters of *ptr and val, that makes lock_stealing hit too much.
and when I change SPIN_THRESHOLD to a small value, system is very much unstable because waiter will enter pv_wait quickly and no one will kick waiter's cpu if
we enter pv_wait twice thanks to the lock_stealing.
So what I do in my pv-qspinlcok patchset V3 is that add if (*ptr == val) in pv_wait. However as I mentioned above, then spurious_wakeup count is too high, that also means our cpu
slice is wasted.

diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 2bbffe4..3482ce9 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -429,14 +429,15 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)

- * Put the lock into the hash table and set the _Q_SLOW_VAL.
- *
- * As this is the same vCPU that will check the _Q_SLOW_VAL value and
- * the hash table later on at unlock time, no atomic instruction is
- * needed.
+ * Put the lock into the hash table and set the _Q_SLOW_VAL later
- WRITE_ONCE(l->locked, _Q_SLOW_VAL);
(void)pv_hash(lock, pn);
+ /*
+ * Match the smp_load_acquire in pv_wait_head_or_lock()
+ * We mush set the _Q_SLOW_VAL after hash.
+ */
+ smp_store_release(&l->locked, _Q_SLOW_VAL);

@@ -454,6 +455,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
struct qspinlock **lp = NULL;
int waitcnt = 0;
int loop;
+ u8 lock_val;

* If pv_kick_node() already advanced our state, we don't need to
@@ -487,7 +489,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)

- if (!lp) { /* ONCE */
+ if (!lp) {
lp = pv_hash(lock, pn);

@@ -517,6 +519,13 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
qstat_inc(qstat_pv_wait_again, waitcnt);

+ * make sure pv_kick_node has hashed the lock, so after pv_wait
+ * if ->locked is not _Q_SLOW_VAL, we can hash the lock again.
+ */
+ if (lp == (struct qspinlock **)1
+ && smp_load_acquire(&l->locked) == _Q_SLOW_VAL)
+ lp = (struct qspinlock **)2;
+ /*
* Pass in the previous node vCPU nmber which is likely to be
* the lock holder vCPU. This additional information may help
* the hypervisor to give more resource to that vCPU so that
@@ -525,13 +534,27 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
pv_wait(&l->locked, _Q_SLOW_VAL, pn->prev_cpu);

+ lock_val = READ_ONCE(l->locked);
+ /* if ->locked is zero, then lock owner has unhashed the lock
+ * if ->locked is _Q_LOCKED_VAL,
+ * 1) pv_kick_node didnot hash the lock, and lp != 0x1
+ * 2) lock stealing, lock owner has unhashed the lock too
+ * 3) race with pv_kick_node, if lp == 2, we know it has hashed
+ * the lock and the lock is unhashed in unlock()
+ * if ->lock is _Q_SLOW_VAL, spurious_wakeup?
+ */
+ if (lock_val != _Q_SLOW_VAL) {
+ if (lock_val == 0 || lp != (struct qspinlock **)1)
+ lp = 0;
+ }

It is a bit hard to verify the correctness of these checks as many variables are involved. Instead, I posted an alternative way to solve this problem by focusing just on the atomic setting of _Q_SLOW_VAL which should be easier to understand and verified. Would you mind testing that patch to see if it can fix the problem that you see?