Re: [External] Re: [PATCH RFC] x86/kvm: Use native qspinlock by default when realtime hinted
From: Liangyan
Date: Fri Jul 04 2025 - 02:12:44 EST
Find one AMD guest(AMD EPYC 9Y24 128-vCPU) to test, it seems about 9%
improvement.
Command: ./Run -c 128 spawn
With virt spin lock:
System Benchmarks Partial Index BASELINE RESULT INDEX
Process Creation 126.0 120449.8 9559.5
========
System Benchmarks Index Score (Partial Only 9559.5
With qspinlock:
System Benchmarks Partial Index BASELINE RESULT INDEX
Process Creation 126.0 131566.8 10441.8
========
System Benchmarks Index Score (Partial Only) 10441.8
Regards,
Liangyan
On 2025/7/3 00:26, Konrad Rzeszutek Wilk wrote:
On Wed, Jul 02, 2025 at 08:23:58PM +0800, Liangyan wrote:
We test that unixbench spawn has big improvement in Intel 8582c 120-CPU
guest vm if switch to qspinlock.
And ARM or AMD?
Command: ./Run -c 120 spawn
Use virt_spin_lock:
System Benchmarks Partial Index BASELINE RESULT INDEX
Process Creation 126.0 71878.4 5704.6
========
System Benchmarks Index Score (Partial Only) 5704.6
Use qspinlock:
System Benchmarks Partial Index BASELINE RESULT INDEX
Process Creation 126.0 173566.6 13775.1
========
System Benchmarks Index Score (Partial Only 13775.1
Regards,
Liangyan
On 2025/7/2 16:19, Bibo Mao wrote:
On 2025/7/2 下午2:42, Liangyan wrote:
When KVM_HINTS_REALTIME is set and KVM_FEATURE_PV_UNHALT is clear,
currently guest will use virt_spin_lock.
Since KVM_HINTS_REALTIME is set, use native qspinlock should be safe
and have better performance than virt_spin_lock.
Just be curious, do you have actual data where native qspinlock has
better performance than virt_spin_lock()?
By my understanding, qspinlock is not friendly with VM. When lock is
released, it is acquired with one by one order in contending queue. If
the first vCPU in contending queue is preempted, the other vCPUs can not
get lock. On physical machine it is almost impossible that CPU
contending lock is preempted.
Regards
Bibo Mao
Signed-off-by: Liangyan <liangyan.peng@xxxxxxxxxxxxx>
---
arch/x86/kernel/kvm.c | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 921c1c783bc1..9080544a4007 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -1072,6 +1072,15 @@ static void kvm_wait(u8 *ptr, u8 val)
*/
void __init kvm_spinlock_init(void)
{
+ /*
+ * Disable PV spinlocks and use native qspinlock when dedicated
pCPUs
+ * are available.
+ */
+ if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
+ pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME
hints\n");
+ goto out;
+ }
+
/*
* In case host doesn't support KVM_FEATURE_PV_UNHALT there is
still an
* advantage of keeping virt_spin_lock_key enabled:
virt_spin_lock() is
@@ -1082,15 +1091,6 @@ void __init kvm_spinlock_init(void)
return;
}
- /*
- * Disable PV spinlocks and use native qspinlock when dedicated
pCPUs
- * are available.
- */
- if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
- pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME
hints\n");
- goto out;
- }
-
if (num_possible_cpus() == 1) {
pr_info("PV spinlocks disabled, single CPU\n");
goto out;