This patch series further filters better vcpu candidate to yield to
in PLE handler. The main idea is to record the preempted vcpus using
preempt notifiers and iterate only those preempted vcpus in the
handler. Note that the vcpus which were in spinloop during pause loop
exit are already filtered.
Thanks Jiannan, Avi for bringing the idea and Gleb, PeterZ for
precious suggestions during the discussion.
Thanks Srikar for suggesting to avoid rcu lock while checking task state
that has improved overcommit cases.
There are basically two approches for the implementation.
Method 1: Uses per vcpu preempt flag (this series).
Method 2: We keep a bitmap of preempted vcpus. using this we can easily
iterate over preempted vcpus.
Note that method 2 needs an extra index variable to identify/map bitmap to
vcpu and it also needs static vcpu allocation.
I am also posting Method 2 approach for reference in case it interests.
Result: decent improvement for kernbench and ebizzy.
base = 3.8.0 + undercommit patches
patched = base + preempt patches
Tested on 32 core (no HT) mx3850 machine with 32 vcpu guest 8GB RAM
--+-----------+-----------+-----------+------------+-----------+
kernbench (exec time in sec lower is beter)
--+-----------+-----------+-----------+------------+-----------+
base stdev patched stdev %improve
--+-----------+-----------+-----------+------------+-----------+
1x 47.0383 4.6977 44.2584 1.2899 5.90986
2x 96.0071 7.1873 91.2605 7.3567 4.94401
3x 164.0157 10.3613 156.6750 11.4267 4.47561
4x 212.5768 23.7326 204.4800 13.2908 3.80888
--+-----------+-----------+-----------+------------+-----------+
no ple kernbench 1x result for reference: 46.056133
--+-----------+-----------+-----------+------------+-----------+
ebizzy (record/sec higher is better)
--+-----------+-----------+-----------+------------+-----------+
base stdev patched stdev %improve
--+-----------+-----------+-----------+------------+-----------+
1x 5609.2000 56.9343 6263.7000 64.7097 11.66833
2x 2071.9000 108.4829 2653.5000 181.8395 28.07085
3x 1557.4167 109.7141 1993.5000 166.3176 28.00043
4x 1254.7500 91.2997 1765.5000 237.5410 40.70532
--+-----------+-----------+-----------+------------+-----------+
no ple ebizzy 1x result for reference : 7394.9 rec/sec
Please let me know if you have any suggestions and comments.
Raghavendra K T (2):
kvm: Record the preemption status of vcpus using preempt notifiers
kvm: Iterate over only vcpus that are preempted
----
include/linux/kvm_host.h | 1 +
virt/kvm/kvm_main.c | 7 +++++++
2 files changed, 8 insertions(+)
Reference patch for Method 2
---8<---
Use preempt bitmap and optimize vcpu iteration using preempt notifiers
From: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
Record the preempted vcpus in a bit map using preempt notifiers.
Add the logic of iterating over only preempted vcpus thus making
vcpu iteration fast.
Thanks Jiannan, Avi for initially proposing patch. Gleb, Peter for
precious suggestions.
Thanks srikar for suggesting to remove rcu lock while checking
task state that helped in reducing overcommit overhead
Not-yet-signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
---
include/linux/kvm_host.h | 7 +++++++
virt/kvm/kvm_main.c | 15 ++++++++++++---
2 files changed, 19 insertions(+), 3 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index cad77fe..8c4a2409 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -252,6 +252,7 @@ struct kvm_vcpu {
bool dy_eligible;
} spin_loop;
#endif
+ int idx;
struct kvm_vcpu_arch arch;
};
@@ -385,6 +386,7 @@ struct kvm {
long mmu_notifier_count;
#endif
long tlbs_dirty;
+ DECLARE_BITMAP(preempt_bitmap, KVM_MAX_VCPUS);
};
#define kvm_err(fmt, ...) \
@@ -413,6 +415,11 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
(vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
idx++)
+#define kvm_for_each_preempted_vcpu(idx, vcpup, kvm, n) \
+ for (idx = find_first_bit(kvm->preempt_bitmap, KVM_MAX_VCPUS); \
+ idx < n && (vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
+ idx = find_next_bit(kvm->preempt_bitmap, KVM_MAX_VCPUS, idx+1))
+
#define kvm_for_each_memslot(memslot, slots) \
for (memslot = &slots->memslots[0]; \
memslot < slots->memslots + KVM_MEM_SLOTS_NUM && memslot->npages;\
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index adc68fe..1db16b3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1770,10 +1770,12 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
struct kvm_vcpu *vcpu;
int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
int yielded = 0;
+ int num_vcpus;
int try = 3;
int pass;
int i;
-
+
+ num_vcpus = atomic_read(&kvm->online_vcpus);
kvm_vcpu_set_in_spin_loop(me, true);
/*
* We boost the priority of a VCPU that is runnable but not
@@ -1783,7 +1785,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
* We approximate round-robin by starting at the last boosted VCPU.
*/
for (pass = 0; pass < 2 && !yielded && try; pass++) {
- kvm_for_each_vcpu(i, vcpu, kvm) {
+ kvm_for_each_preempted_vcpu(i, vcpu, kvm, num_vcpus) {
if (!pass && i <= last_boosted_vcpu) {
i = last_boosted_vcpu;
continue;
@@ -1878,6 +1880,7 @@ static int create_vcpu_fd(struct kvm_vcpu *vcpu)
static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
{
int r;
+ int curr_idx;
struct kvm_vcpu *vcpu, *v;
vcpu = kvm_arch_vcpu_create(kvm, id);
@@ -1916,7 +1919,9 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
goto unlock_vcpu_destroy;
}
- kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
+ curr_idx = atomic_read(&kvm->online_vcpus);
+ kvm->vcpus[curr_idx] = vcpu;
+ vcpu->idx = curr_idx;
smp_wmb();
atomic_inc(&kvm->online_vcpus);
@@ -2902,6 +2907,7 @@ struct kvm_vcpu *preempt_notifier_to_vcpu(struct preempt_notifier *pn)
static void kvm_sched_in(struct preempt_notifier *pn, int cpu)
{
struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
+ clear_bit(vcpu->idx, vcpu->kvm->preempt_bitmap);
kvm_arch_vcpu_load(vcpu, cpu);
}
@@ -2911,6 +2917,9 @@ static void kvm_sched_out(struct preempt_notifier *pn,
{
struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
+ if (current->state == TASK_RUNNING)
+ set_bit(vcpu->idx, vcpu->kvm->preempt_bitmap);
+
kvm_arch_vcpu_put(vcpu);
}
--
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