This is an RFC since I have not done any comparison with the approach
using for_each_set_bit() which can be seen in Avi's work.
Takuya
---
We did a simple test to see which requests we would get at the same time
in vcpu_enter_guest() and got the following numbers:
|...........|...............|........|...............|.|
(N) (E) (S) (ES) others
22.3% 30.7% 16.0% 29.5% 1.4%
(N) : Nothing
(E) : Only KVM_REQ_EVENT
(S) : Only KVM_REQ_STEAL_UPDATE
(ES): Only KVM_REQ_EVENT and KVM_REQ_STEAL_UPDATE
* Note that the exact numbers can change for other guests.
This motivated us to optimize the following code in vcpu_enter_guest():
if (vcpu->requests) { /** (1) **/
...
if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu)) /** (2) **/
record_steal_time(vcpu);
...
}
if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
...
}
- For case (E), we do kvm_check_request() for every request other than
KVM_REQ_EVENT in the block (1) and always get false.
- For case (S) and (ES), the only difference from case (E) is that we
get true for KVM_REQ_STEAL_UPDATE.
This means that we were wasting a lot of time for the many if branches
in the block (1) even for these trivial three cases which dominated more
than 75% in total.
This patch mitigates the issue as follows:
- For case (E), we change the first if condition to
if (vcpu->requests & ~(1 << KVM_REQ_EVENT)) /** (1') **/
so that we can skip the block completely.
- For case (S) and (ES), we move the check (2) upwards, out of the
block (1), to clear the KVM_REQ_STEAL_UPDATE flag before doing the
check (1').
Although this adds one if branch for case (N), the fact that steal
update occurs frequently enough except when we give each vcpu a
dedicated core justifies its tiny cost.
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@xxxxxxxxxxxxx>
---
[My email address change is not a mistake.]
arch/x86/kvm/x86.c | 11 ++++++++---
1 files changed, 8 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fc2a0a1..e351902 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5233,7 +5233,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
vcpu->run->request_interrupt_window;
bool req_immediate_exit = 0;
- if (vcpu->requests) {
+ /*
+ * Getting KVM_REQ_STEAL_UPDATE alone is so common that clearing it now
+ * will hopefully result in skipping the following checks.
+ */
+ if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
+ record_steal_time(vcpu);
+
+ if (vcpu->requests & ~(1 << KVM_REQ_EVENT)) {
if (kvm_check_request(KVM_REQ_MMU_RELOAD, vcpu))
kvm_mmu_unload(vcpu);
if (kvm_check_request(KVM_REQ_MIGRATE_TIMER, vcpu))
@@ -5267,8 +5274,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
r = 1;
goto out;
}
- if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
- record_steal_time(vcpu);
if (kvm_check_request(KVM_REQ_NMI, vcpu))
process_nmi(vcpu);
req_immediate_exit =
--
1.7.5.4
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