[PATCH] x86/paravirt: add backoff mechanism to virt_spin_lock

[PATCH] x86/paravirt: add backoff mechanism to virt_spin_lock

[Wangyang Guo]

When multiple threads waiting for lock at the same time, once lock owner
releases the lock, waiters will see lock available and all try to lock,
which may cause an expensive CAS storm.

Binary exponential backoff is introduced. As try-lock attempt increases,
there is more likely that a larger number threads compete for the same
lock, so increase wait time in exponential.

The optimization can improves SpecCPU2017 502.gcc_r benchmark by ~4% for
288 cores VM on Intel Xeon 6 E-cores platform.

For micro benchmark, the patch can have significant performance gain
in high contention case. Slight regression is found in some of mid-
conetented cases because the last waiter might take longer to check
unlocked. No changes to low contented scenario as expected.

Micro Bench: https://github.com/guowangy/kernel-lock-bench
Test Platform: Xeon 8380L
First Row: critical section length
First Col: CPU core number
Values: backoff vs linux-6.15, throughput based, higher is better

non-critical-length: 1
       0     1     2     4     8    16    32    64   128
1   1.01  1.00  1.00  1.00  1.01  1.01  1.01  1.01  1.00
2   1.02  1.01  1.02  0.97  1.02  1.05  1.01  1.00  1.01
4   1.15  1.20  1.14  1.11  1.34  1.26  0.99  0.93  0.98
8   1.59  1.71  1.18  1.80  1.95  1.45  1.05  0.99  1.17
16  1.04  1.37  1.08  1.31  1.85  1.50  1.24  0.99  1.24
32  1.24  1.36  1.23  1.40  1.50  1.86  1.45  1.18  1.48
64  1.12  1.24  1.11  1.31  1.34  1.37  2.01  1.60  1.43

non-critical-length: 32
       0     1     2     4     8    16    32    64   128
1   1.00  1.00  1.00  1.00  1.00  0.99  1.00  1.00  1.01
2   1.00  1.00  1.00  1.00  1.00  1.00  1.00  0.99  1.00
4   1.12  1.25  1.09  1.07  1.12  1.16  1.13  1.16  1.09
8   1.02  1.16  1.03  1.02  1.04  1.07  1.04  0.99  0.98
16  0.97  0.95  0.84  0.96  0.99  0.98  0.98  1.01  1.03
32  1.05  1.03  0.87  1.05  1.25  1.16  1.25  1.30  1.27
64  1.83  1.10  1.07  1.02  1.19  1.18  1.21  1.14  1.13

non-critical-length: 128
       0     1     2     4     8    16    32    64   128
1   1.00  1.00  1.00  1.00  1.00  1.00  1.00  1.00  1.00
2   0.99  1.02  1.00  1.00  1.00  1.00  1.00  1.00  1.00
4   0.98  0.99  1.00  1.00  0.99  1.04  0.99  0.99  1.02
8   1.08  1.08  1.08  1.07  1.15  1.12  1.03  0.94  1.00
16  1.00  1.00  1.00  1.01  1.01  1.01  1.36  1.06  1.02
32  1.07  1.08  1.07  1.07  1.09  1.10  1.22  1.36  1.25
64  1.03  1.04  1.04  1.06  1.13  1.18  0.82  1.02  1.14

Reviewed-by: Tianyou Li <tianyou.li@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
---
 arch/x86/include/asm/qspinlock.h | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index 68da67df304d..ac6e1bbd9ba4 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -87,7 +87,7 @@ DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key);
 #define virt_spin_lock virt_spin_lock
 static inline bool virt_spin_lock(struct qspinlock *lock)
 {
-	int val;
+	int val, locked;
 
 	if (!static_branch_likely(&virt_spin_lock_key))
 		return false;
@@ -98,11 +98,33 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
 	 * horrible lock 'holder' preemption issues.
 	 */
 
+#define MAX_BACKOFF 64
+	int backoff = 1;
+
  __retry:
 	val = atomic_read(&lock->val);
+	locked = val;
+
+	if (locked || !atomic_try_cmpxchg(&lock->val, &val, _Q_LOCKED_VAL)) {
+		int spin_count = backoff;
+
+		while (spin_count--)
+			cpu_relax();
+
+		/*
+		 * Here not locked means lock tried, but fails.
+		 *
+		 * When multiple threads waiting for lock at the same time,
+		 * once lock owner releases the lock, waiters will see lock available
+		 * and all try to lock, which may cause an expensive CAS storm.
+		 *
+		 * Binary exponential backoff is introduced. As try-lock attempt
+		 * increases, there is more likely that a larger number threads
+		 * compete for the same lock, so increase wait time in exponential.
+		 */
+		if (!locked)
+			backoff = (backoff < MAX_BACKOFF) ? backoff << 1 : backoff;
 
-	if (val || !atomic_try_cmpxchg(&lock->val, &val, _Q_LOCKED_VAL)) {
-		cpu_relax();
 		goto __retry;
 	}
 
-- 
2.43.5