On 19.10.21 19:54, Halil Pasic wrote:
The idea behind kicked mask is that we should not re-kick a vcpu
from __airqs_kick_single_vcpu() that is already in the middle of
being kicked by the same function.
If however the vcpu that was idle before when the idle_mask was
examined, is not idle any more after the kicked_mask is set, that
means that we don't need to kick, and that we need to clear the
bit we just set because we may be beyond the point where it would
get cleared in the wake-up process. Since the time window is short,
this is probably more a theoretical than a practical thing: the race
window is small.
To get things harmonized let us also move the clear from vcpu_pre_run()
to __unset_cpu_idle().
Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
Fixes: 9f30f6216378 ("KVM: s390: add gib_alert_irq_handler()")
Before releasing something like this, where none of us is sure if
it really saves cpu cost, I'd prefer to run some measurement with
the whole kicked_mask logic removed and to compare the number of
vcpu wake ups with the number of interrupts to be processed by
the gib alert mechanism in a slightly over committed host while
driving with Matthews test load.
A similar run can be done with this code.
---
arch/s390/kvm/interrupt.c | 7 ++++++-
arch/s390/kvm/kvm-s390.c | 2 --
2 files changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 2245f4b8d362..3c80a2237ef5 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -426,6 +426,7 @@ static void __unset_cpu_idle(struct kvm_vcpu *vcpu)
{
kvm_s390_clear_cpuflags(vcpu, CPUSTAT_WAIT);
clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.idle_mask);
+ clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.gisa_int.kicked_mask);
}
static void __reset_intercept_indicators(struct kvm_vcpu *vcpu)
@@ -3064,7 +3065,11 @@ static void __airqs_kick_single_vcpu(struct kvm *kvm, u8 deliverable_mask)
/* lately kicked but not yet running */
if (test_and_set_bit(vcpu_idx, gi->kicked_mask))
return;
- kvm_s390_vcpu_wakeup(vcpu);
+ /* if meanwhile not idle: clear and don't kick */
+ if (test_bit(vcpu_idx, kvm->arch.idle_mask))
+ kvm_s390_vcpu_wakeup(vcpu);
+ else
+ clear_bit(vcpu_idx, gi->kicked_mask);
return;
}
}
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 1c97493d21e1..6b779ef9f5fb 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4067,8 +4067,6 @@ static int vcpu_pre_run(struct kvm_vcpu *vcpu)
kvm_s390_patch_guest_per_regs(vcpu);
}
- clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.gisa_int.kicked_mask);
-
vcpu->arch.sie_block->icptcode = 0;
cpuflags = atomic_read(&vcpu->arch.sie_block->cpuflags);
VCPU_EVENT(vcpu, 6, "entering sie flags %x", cpuflags);
