v3->v4:
- Patch 1: add comment about possible racing condition in PV unlock.
- Patch 2: simplified the pv_pending_lock() function as suggested by
Davidlohr.
- Move PV unlock optimization patch forward to patch 4 & rerun
performance test.
v2->v3:
- Moved deferred kicking enablement patch forward & move back
the kick-ahead patch to make the effect of kick-ahead more visible.
- Reworked patch 6 to make it more readable.
- Reverted back to use state as a tri-state variable instead of
adding an additional bistate variable.
- Added performance data for different values of PV_KICK_AHEAD_MAX.
- Add a new patch to optimize PV unlock code path performance.
v1->v2:
- Take out the queued unfair lock patches
- Add a patch to simplify the PV unlock code
- Move pending bit and statistics collection patches to the front
- Keep vCPU kicking in pv_kick_node(), but defer it to unlock time
when appropriate.
- Change the wait-early patch to use adaptive spinning to better
balance the difference effect on normal and over-committed guests.
- Add patch-to-patch performance changes in the patch commit logs.
This patchset tries to improve the performance of both normal and
over-commmitted VM guests. The kick-ahead and adaptive spinning
patches are inspired by the "Do Virtual Machines Really Scale?" blog
from Sanidhya Kashyap.
Patch 1 simplifies the unlock code by doing unconditional vCPU kick
when _Q_SLOW_VAL is set as the chance of spurious wakeup showing
up in the statistical data that I collected was very low (1 or 2
occasionally).
Patch 2 adds pending bit support to pvqspinlock improving performance
at light load.
Patch 3 allows the collection of various count data that are useful
to see what is happening in the system. They do add a bit of overhead
when enabled slowing performance a tiny bit.
Patch 4 optimizes the PV unlock code path performance for x86-64
architecture.
Patch 5 is an enablement patch for deferring vCPU kickings from the
lock side to the unlock side.
Patch 6 enables multiple vCPU kick-ahead's at unlock time, outside of
the critical section which can improve performance in overcommitted
guests and sometime even in normal guests.
Patch 7 enables adaptive spinning in the queue nodes. This patch can
lead to pretty big performance increase in over-committed guest at
the expense of a slight performance hit in normal guests.
Patches 2 & 4 improves performance of common uncontended and lightly
contended cases. Patches 5-7 are for improving performance in
over-committed VM guests.
Performance measurements were done on a 32-CPU Westmere-EX and
Haswell-EX systems. The Westmere-EX system got the most performance
gain from patch 5, whereas the Haswell-EX system got the most gain
from patch 6 for over-committed guests.
The table below shows the Linux kernel build times for various
values of PV_KICK_AHEAD_MAX on an over-committed 48-vCPU guest on
the Westmere-EX system:
PV_KICK_AHEAD_MAX Patches 1-5 Patches 1-6
----------------- ----------- -----------
1 9m46.9s 11m10.1s
2 9m40.2s 10m08.3s
3 9m36.8s 9m49.8s
4 9m35.9s 9m38.7s
5 9m35.1s 9m33.0s
6 9m35.7s 9m28.5s
With patches 1-5, the performance wasn't very sensitive to different
PV_KICK_AHEAD_MAX values. Adding patch 6 into the mix, however, changes
the picture quite dramatically. There is a performance regression if
PV_KICK_AHEAD_MAX is too small. Starting with a value of 4, increasing
PV_KICK_AHEAD_MAX only gets us a minor benefit.
Waiman Long (7):
locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL
locking/pvqspinlock: Add pending bit support
locking/pvqspinlock: Collect slowpath lock statistics
locking/pvqspinlock, x86: Optimize PV unlock code path
locking/pvqspinlock: Enable deferment of vCPU kicking to unlock call
locking/pvqspinlock: Allow vCPUs kick-ahead
locking/pvqspinlock: Queue node adaptive spinning
arch/x86/Kconfig | 7 +
arch/x86/include/asm/qspinlock_paravirt.h | 59 +++
kernel/locking/qspinlock.c | 38 ++-
kernel/locking/qspinlock_paravirt.h | 546 +++++++++++++++++++++++++++--
4 files changed, 612 insertions(+), 38 deletions(-)
v3-to-v4 diff:
arch/x86/include/asm/qspinlock_paravirt.h | 5 +--
kernel/locking/qspinlock_paravirt.h | 45 +++++++++++++++++-----------
2 files changed, 29 insertions(+), 21 deletions(-)
diff --git a/arch/x86/include/asm/qspinlock_paravirt.h
b/arch/x86/include/asm/qspinlock_paravirt.h
index 46f0f82..3001972 100644
--- a/arch/x86/include/asm/qspinlock_paravirt.h
+++ b/arch/x86/include/asm/qspinlock_paravirt.h
@@ -9,10 +9,10 @@
*/
#ifdef CONFIG_64BIT
-PV_CALLEE_SAVE_REGS_THUNK(pv_queued_spin_unlock_slowpath);
+PV_CALLEE_SAVE_REGS_THUNK(__pv_queued_spin_unlock_slowpath);
#define __pv_queued_spin_unlock __pv_queued_spin_unlock
#define PV_UNLOCK "__raw_callee_save___pv_queued_spin_unlock"
-#define PV_UNLOCK_SLOWPATH
"__raw_callee_save_pv_queued_spin_unlock_slowpath"
+#define PV_UNLOCK_SLOWPATH
"__raw_callee_save___pv_queued_spin_unlock_slowpath"
/*
* Optimized assembly version of __raw_callee_save___pv_queued_spin_unlock
@@ -46,7 +46,6 @@ asm(".pushsection .text;"
"jne .slowpath;"
"pop %rdx;"
"ret;"
- "nop;"
".slowpath: "
"push %rsi;"
"movzbl %al,%esi;"
diff --git a/kernel/locking/qspinlock_paravirt.h
b/kernel/locking/qspinlock_paravirt.h
index 56c3717..d04911b 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -463,16 +463,16 @@ static int pv_pending_lock(struct qspinlock *lock, u32
val)
/*
* wait for in-progress pending->locked hand-overs
*/
- if (val == _Q_PENDING_VAL) {
- while (((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) &&
- loop--)
- cpu_relax();
+ while ((val == _Q_PENDING_VAL) && loop) {
+ cpu_relax();
+ val = atomic_read(&lock->val);
+ loop--;
}
/*
* trylock || pending
*/
- for (;;) {
+ for (;; loop--) {
if (val & ~_Q_LOCKED_MASK)
goto queue;
new = _Q_LOCKED_VAL;
@@ -481,7 +481,7 @@ static int pv_pending_lock(struct qspinlock *lock, u32 val)
old = atomic_cmpxchg(&lock->val, val, new);
if (old == val)
break;
- if (loop-- <= 0)
+ if (!loop)
goto queue;
}
@@ -490,16 +490,18 @@ static int pv_pending_lock(struct qspinlock *lock, u32
val)
/*
* We are pending, wait for the owner to go away.
*/
- while (((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_MASK)
- && (loop-- > 0))
- cpu_relax();
-
- if (!(val & _Q_LOCKED_MASK)) {
- clear_pending_set_locked(lock);
- goto gotlock;
+ for (; loop; loop--, cpu_relax()) {
+ val = smp_load_acquire(&lock->val.counter);
+ if (!(val & _Q_LOCKED_MASK)) {
+ clear_pending_set_locked(lock);
+ pvstat_inc(pvstat_pend_lock);
+ goto gotlock;
+ }
}
+
/*
- * Clear the pending bit and fall back to queuing
+ * Fail to acquire the lock within the spinning period,
+ * so clear the pending bit and fall back to queuing.
*/
clear_pending(lock);
pvstat_inc(pvstat_pend_fail);
@@ -719,11 +721,11 @@ static void pv_wait_head(struct qspinlock *lock, struct
mcs_spinlock *node)
}
/*
- * PV version of the unlock fastpath and slowpath functions to be used
- * in stead of queued_spin_unlock().
+ * PV versions of the unlock fastpath and slowpath functions to be used
+ * instead of queued_spin_unlock().
*/
__visible void
-pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 lockval)
+__pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 lockval)
{
struct __qspinlock *l = (void *)lock;
struct pv_node *node, *next;
@@ -771,6 +773,13 @@ pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8
lockval)
/*
* At this point the memory pointed at by lock can be freed/reused,
* however we can still use the pv_node to kick the CPU.
+ *
+ * As smp_store_release() is not a full barrier, adding a check to
+ * the node->state doesn't guarantee the checking is really done
+ * after clearing the lock byte since they are in 2 separate
+ * cachelines and so hardware can reorder them. So either we insert
+ * memory barrier here and in the corresponding pv_wait_head()
+ * function or we do an unconditional kick which is what is done here.
*/
pvstat_inc(pvstat_unlock_kick);
pv_kick(node->cpu);
@@ -803,6 +812,6 @@ __visible void __pv_queued_spin_unlock(struct qspinlock
*lock)
if (likely(lockval == _Q_LOCKED_VAL))
return;
- pv_queued_spin_unlock_slowpath(lock, lockval);
+ __pv_queued_spin_unlock_slowpath(lock, lockval);
}
#endif /* __pv_queued_spin_unlock */
--
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