linux-kernel - [PATCH] KVM: halt polling: Make the adjustment of polling time clearer

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Date:   Thu, 20 May 2021 11:05:29 +0800
From:   Keqian Zhu <zhukeqian1@...wei.com>
To:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-doc@...r.kernel.org>, <kvm@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Jonathan Corbet" <corbet@....net>, Marc Zyngier <maz@...nel.org>
CC:     <wanghaibin.wang@...wei.com>, <zhang.zhanghailiang@...wei.com>,
        <gaojinhao@...wei.com>
Subject: [PATCH] KVM: halt polling: Make the adjustment of polling time clearer

When we have "block_ns > halt_poll_ns" and "block_ns < max_halt_poll_ns",
then "halt_poll_ns < max_halt_poll_ns" is true, so we can drop this extra
condition.

We want to make sure halt_poll_ns is not zero before shrinking it. Put
the condition in shrinking primitive can make code clearer.

None functional change.

Signed-off-by: Keqian Zhu <zhukeqian1@...wei.com>
---
 Documentation/virt/kvm/halt-polling.rst | 21 ++++++++++-----------
 virt/kvm/kvm_main.c                     | 11 ++++++-----
 2 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/Documentation/virt/kvm/halt-polling.rst b/Documentation/virt/kvm/halt-polling.rst
index 4922e4a15f18..d9f699395a7f 100644
--- a/Documentation/virt/kvm/halt-polling.rst
+++ b/Documentation/virt/kvm/halt-polling.rst
@@ -47,17 +47,16 @@ Thus this is a per vcpu (or vcore) value.
 During polling if a wakeup source is received within the halt polling interval,
 the interval is left unchanged. In the event that a wakeup source isn't
 received during the polling interval (and thus schedule is invoked) there are
-two options, either the polling interval and total block time[0] were less than
-the global max polling interval (see module params below), or the total block
-time was greater than the global max polling interval.
-
-In the event that both the polling interval and total block time were less than
-the global max polling interval then the polling interval can be increased in
-the hope that next time during the longer polling interval the wake up source
-will be received while the host is polling and the latency benefits will be
-received. The polling interval is grown in the function grow_halt_poll_ns() and
-is multiplied by the module parameters halt_poll_ns_grow and
-halt_poll_ns_grow_start.
+two options, either the total block time[0] were less than the global max
+polling interval (see module params below), or the total block time was greater
+than the global max polling interval.
+
+In the event that the total block time were less than the global max polling
+interval then the polling interval can be increased in the hope that next time
+during the longer polling interval the wake up source will be received while the
+host is polling and the latency benefits will be received. The polling interval
+is grown in the function grow_halt_poll_ns() and is multiplied by the module
+parameters halt_poll_ns_grow and halt_poll_ns_grow_start.
 
 In the event that the total block time was greater than the global max polling
 interval then the host will never poll for long enough (limited by the global
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6b4feb92dc79..13a9996c4ccb 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2906,6 +2906,9 @@ static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
 	unsigned int old, val, shrink;
 
 	old = val = vcpu->halt_poll_ns;
+	if (!old)
+		return;
+
 	shrink = READ_ONCE(halt_poll_ns_shrink);
 	if (shrink == 0)
 		val = 0;
@@ -3003,12 +3006,10 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 			if (block_ns <= vcpu->halt_poll_ns)
 				;
 			/* we had a long block, shrink polling */
-			else if (vcpu->halt_poll_ns &&
-					block_ns > vcpu->kvm->max_halt_poll_ns)
+			else if (block_ns > vcpu->kvm->max_halt_poll_ns)
 				shrink_halt_poll_ns(vcpu);
-			/* we had a short halt and our poll time is too small */
-			else if (vcpu->halt_poll_ns < vcpu->kvm->max_halt_poll_ns &&
-					block_ns < vcpu->kvm->max_halt_poll_ns)
+			/* we had a short block, grow polling */
+			else if (block_ns < vcpu->kvm->max_halt_poll_ns)
 				grow_halt_poll_ns(vcpu);
 		} else {
 			vcpu->halt_poll_ns = 0;
-- 
2.19.1