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<title>kubernetes 中 Evicted pod 是如何产生的 · 田飞雨</title>
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kubelet 启动流程分析
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kubelet 中垃圾回收机制的设计与实现
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<li class="chapter active" data-level="1.2.25" data-path="k8s_evicted.html">
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<b>1.2.25.</b>
kubernetes 中 Evicted pod 是如何产生的
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<b>1.2.26.</b>
Kubernetes 中 Descheduler 组件的使用与扩展
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在 minkube 上部署 knative
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golang 中函数使用值返回与指针返回的区别，底层原理分析
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</li>
<li><a href="#kubelet-驱逐实例时与资源处理相关的已知问题">kubelet 驱逐实例时与资源处理相关的已知问题</a></li>
<li><a href="#驱逐实例未被删除原因分析">驱逐实例未被删除原因分析</a></li>
<li><a href="#解决方案">解决方案</a></li>
<li><a href="#总结">总结</a></li>
</ul>
<h3 id="线上被驱逐实例数据"><a name="线上被驱逐实例数据" class="anchor-navigation-ex-anchor" href="#线上被驱逐实例数据"><i class="fa fa-link" aria-hidden="true"></i></a>线上被驱逐实例数据</h3>
<p>最近在线上发现很多实例处于 Evicted 状态，通过 pod yaml 可以看到实例是因为节点资源不足被驱逐，但是这些实例并没有被自动清理，平台的大部分用户在操作时看到服务下面出现 Evicted 实例时会以为服务有问题或者平台有问题的错觉，影响了用户的体验。而这部分 Evicted 状态的 Pod 在底层关联的容器其实已经被销毁了，对用户的服务也不会产生什么影响，也就是说只有一个 Pod 空壳在 k8s 中保存着，但需要人为手动清理。本文会分析为什么为产生 Evicted 实例、为什么 Evicted 实例没有被自动清理以及如何进行自动清理。</p>
<blockquote>
<p>kubernetes 版本：v1.17</p>
</blockquote>
<pre class="language-"><code>$ kubectl get pod | grep -i Evicted
cloud-1023955-84421-49604-5-deploy-c-7748f8fd8-hjqsh 0/1 Evicted 0 73d
cloud-1023955-84421-49604-5-deploy-c-7748f8fd8-mzd8x 0/1 Evicted 0 81d
cloud-1237162-276467-199844-2-deploy-7bdc7c98b6-26r2r 0/1 Evicted 0 18d
</code></pre><p>Evicted 实例状态：</p>
<pre class="language-"><code>status:
message: 'Pod The node had condition: [DiskPressure]. '
phase: Failed
reason: Evicted
startTime: "2021-09-14T10:42:32Z"
</code></pre><h3 id="实例被驱逐的原因"><a name="实例被驱逐的原因" class="anchor-navigation-ex-anchor" href="#实例被驱逐的原因"><i class="fa fa-link" aria-hidden="true"></i></a>实例被驱逐的原因</h3>
<p>kubelet 默认会配置节点资源不足时驱逐实例的策略，当节点资源不足时 k8s 会停止该节点上实例并在其他节点启动新实例，在某些情况下也可通过配置 <code>--eviction-hard=</code> 参数为空来禁用驱逐策略，在之前的生产环境中我们也确实这么做了。</p>
<h4 id="节点资源不足导致实例被驱逐"><a name="节点资源不足导致实例被驱逐" class="anchor-navigation-ex-anchor" href="#节点资源不足导致实例被驱逐"><i class="fa fa-link" aria-hidden="true"></i></a>节点资源不足导致实例被驱逐</h4>
<p>k8s 中产生 Evicted 状态实例主要是因为节点资源不足实例主动被驱逐导致的，kubelet eviction_manager 模块会定期检查节点内存使用率、inode 使用率、磁盘使用率、pid 等资源，根据 kubelet 的配置当使用率达到一定阈值后会先回收可以回收的资源，若回收后资源使用率依然超过阈值则进行驱逐实例操作。</p>
<table>
<thead>
<tr>
<th>Eviction Signal</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>memory.available</td>
<td>memory.available := node.status.capacity[memory] - node.stats.memory.workingSet</td>
</tr>
<tr>
<td>nodefs.available</td>
<td>nodefs.available := node.stats.fs.available</td>
</tr>
<tr>
<td>nodefs.inodesFree</td>
<td>nodefs.inodesFree := node.stats.fs.inodesFree</td>
</tr>
<tr>
<td>imagefs.available</td>
<td>imagefs.available := node.stats.runtime.imagefs.available</td>
</tr>
<tr>
<td>imagefs.inodesFree</td>
<td>imagefs.inodesFree := node.stats.runtime.imagefs.inodesFree</td>
</tr>
<tr>
<td>pid.available</td>
<td>pid.available := node.stats.rlimit.maxpid - node.stats.rlimit.curproc</td>
</tr>
</tbody>
</table>
<p>kubelet 中 pod 的 stats 数据一部分是通过 cAdvisor 接口获取到的，一部分是通过 CRI runtimes 的接口获取到的。</p>
<p><strong>memory.available</strong>：当前节点可用内存，计算方式为 cgroup memory 子系统中 memory.usage_in_bytes 中的值减去 memory.stat 中 total_inactive_file 的值；
<strong>nodefs.available</strong>：nodefs 包含 kubelet 配置中 <code>--root-dir</code> 指定的文件分区和 /var/lib/kubelet/ 所在的分区磁盘使用率；
<strong>nodefs.inodesFree</strong>：nodefs.available 分区的 inode 使用率；
<strong>imagefs.available：</strong>镜像所在分区磁盘使用率；
<strong>imagefs.inodesFree：</strong>镜像所在分区磁盘inode使用率；
<strong>pid.available：</strong><code>/proc/sys/kernel/pid_max</code> 中的值为系统最大可用 pid 数；</p>
<p>kubelet 可以通过参数 <code>--eviction-hard</code> 来配置以上几个参数的阈值，该参数默认值为 <code>imagefs.available<15%,memory.available<100Mi,nodefs.available<10%,nodefs.inodesFree<5%</code>，当达到阈值时会驱逐节点上的容器。</p>
<h3 id="kubelet-驱逐实例时与资源处理相关的已知问题"><a name="kubelet-驱逐实例时与资源处理相关的已知问题" class="anchor-navigation-ex-anchor" href="#kubelet-驱逐实例时与资源处理相关的已知问题"><i class="fa fa-link" aria-hidden="true"></i></a>kubelet 驱逐实例时与资源处理相关的已知问题</h3>
<p><strong>1、kubelet 不会实时感知到节点内存数据的变化</strong></p>
<p>kubelet 定期通过 cadvisor 接口采集节点内存使用数据，当节点短时间内内存使用率突增，此时 kubelet 无法感知到也不会有 MemoryPressure 相关事件，但依然会调用 OOMKiller 停止容器。可以通过为 kubelet 配置 <code>--kernel-memcg-notification</code> 参数启用 memcg api，当触发memory 使用率阈值时 memcg 会主动进行通知；</p>
<p>memcg 主动通知的功能是 cgroup 中已有的，kubelet 会在 <code>/sys/fs/cgroup/memory/cgroup.event_control</code> 文件中写入 memory.available 的阈值，而阈值与 inactive_file 文件的大小有关系，kubelet 也会定期更新阈值，当 memcg 使用率达到配置的阈值后会主动通知 kubelet，kubelet 通过 epoll 机制来接收通知。</p>
<p><strong>2、kubelet</strong> <strong>memory.available 不会计算 active page</strong></p>
<p>kubelet 通过内存使用率驱逐实例时，内存使用率数据包含了 page cache 中 active_file 的数据，在某些场景下会因 page cache 过高导致内存使用率超过阈值会造成实例被驱逐，</p>
<p>由于在内存紧张时 inactive_file 会被内核首先回收，但在内存不足时，active_file 也会被内核进行回收，社区对此机制也有一些疑问，针对内核回收内存的情况比较复杂，社区暂时还未进行回应，详情可以参考 <a href="https://github.com/kubernetes/kubernetes/issues/43916" target="_blank">kubelet counts active page cache against memory.available (maybe it shouldn't?)</a>。</p>
<p>kubelet 计算节点可用内存的方式如下：</p>
<pre class="language-"><code>#!/bin/bash
#!/usr/bin/env bash
# This script reproduces what the kubelet does
# to calculate memory.available relative to root cgroup.
# current memory usage
memory_capacity_in_kb=$(cat /proc/meminfo | grep MemTotal | awk '{print $2}')
memory_capacity_in_bytes=$((memory_capacity_in_kb * 1024))
memory_usage_in_bytes=$(cat /sys/fs/cgroup/memory/memory.usage_in_bytes)
memory_total_inactive_file=$(cat /sys/fs/cgroup/memory/memory.stat | grep total_inactive_file | awk '{print $2}')
memory_working_set=${memory_usage_in_bytes}
if [ "$memory_working_set" -lt "$memory_total_inactive_file" ];
then
memory_working_set=0
else
memory_working_set=$((memory_usage_in_bytes - memory_total_inactive_file))
fi
memory_available_in_bytes=$((memory_capacity_in_bytes - memory_working_set))
memory_available_in_kb=$((memory_available_in_bytes / 1024))
memory_available_in_mb=$((memory_available_in_kb / 1024))
echo "memory.capacity_in_bytes $memory_capacity_in_bytes"
echo "memory.usage_in_bytes $memory_usage_in_bytes"
echo "memory.total_inactive_file $memory_total_inactive_file"
echo "memory.working_set $memory_working_set"
echo "memory.available_in_bytes $memory_available_in_bytes"
echo "memory.available_in_kb $memory_available_in_kb"
echo "memory.available_in_mb $memory_available_in_mb"
</code></pre><h3 id="驱逐实例未被删除原因分析"><a name="驱逐实例未被删除原因分析" class="anchor-navigation-ex-anchor" href="#驱逐实例未被删除原因分析"><i class="fa fa-link" aria-hidden="true"></i></a>驱逐实例未被删除原因分析</h3>
<p>源码中对于 Statefulset 和 DaemonSet 会自动删除 Evicted 实例，但是对于 Deployment 不会自动删除。阅读了部分官方文档以及 issue，暂未找到官方对 Deployment Evicted 实例未删除原因给出解释。</p>
<p><strong>statefulset：</strong>
pkg/controller/statefulset/stateful_set_control.go</p>
<pre class="language-"><code> // Examine each replica with respect to its ordinal
for i := range replicas {
// delete and recreate failed pods
if isFailed(replicas[i]) {
ssc.recorder.Eventf(set, v1.EventTypeWarning, "RecreatingFailedPod",
"StatefulSet %s/%s is recreating failed Pod %s",
set.Namespace,
set.Name,
replicas[i].Name)
if err := ssc.podControl.DeleteStatefulPod(set, replicas[i]); err != nil {
return &status, err
}
if getPodRevision(replicas[i]) == currentRevision.Name {
status.CurrentReplicas--
}
if getPodRevision(replicas[i]) == updateRevision.Name {
status.UpdatedReplicas--
}
......
</code></pre><p><strong>daemonset：</strong>
pkg/controller/daemon/daemon_controller.go</p>
<pre class="language-"><code>func (dsc *DaemonSetsController) podsShouldBeOnNode(
......
) (nodesNeedingDaemonPods, podsToDelete []string) {
......
switch {
......
case shouldContinueRunning:
......
for _, pod := range daemonPods {
if pod.DeletionTimestamp != nil {
continue
}
if pod.Status.Phase == v1.PodFailed {
// This is a critical place where DS is often fighting with kubelet that rejects pods.
// We need to avoid hot looping and backoff.
backoffKey := failedPodsBackoffKey(ds, node.Name)