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测试时关注到bw_costly_链
因为和iface有关。猜测这个链是动态生成的。
开关数据业务测试，果然关闭数据业务后，bw_OUTPUT中不再会调用bw_costly_rmnet_data3，也没有bw_costly_rmnet_data3这个链了。
再次打开数据业务后出现了bw_costly_rmnet_data2。说明上次上网的rmnet_data3口，这此上网的rmnet_data2。

:/ # iptables -t filter -nvL bw_OUTPUT
Chain bw_OUTPUT (1 references)pkts bytes target     prot opt in     out     source               destination24  1909 bw_global_alert  all  --  *      *       0.0.0.0/0            0.0.0.0/02   120 bw_costly_rmnet_data3  all  --  *      rmnet_data3  0.0.0.0/0            0.0.0.0/0

是netd模块自己检测上网状态后，自己添加的吗？

查看代码是netd的BandwidthController生成。

int BandwidthController::setInterfaceQuota(const std::string& iface, int64_t maxBytes) {const std::string& cost = iface;
..../* Insert ingress quota. */auto it = mQuotaIfaces.find(iface);if (it != mQuotaIfaces.end()) {if (int res = updateQuota(cost, maxBytes)) {ALOGE("Failed update quota for %s", iface.c_str());removeInterfaceQuota(iface);return res;}it->second.quota = maxBytes;return 0;}const std::string chain = "bw_costly_" + iface;const int ruleInsertPos = (mGlobalAlertBytes) ? 2 : 1;std::vector<std::string> cmds = {"*filter",StringPrintf(":%s -", chain.c_str()),StringPrintf("-A %s -j bw_penalty_box", chain.c_str()),StringPrintf("-I bw_INPUT %d -i %s -j %s", ruleInsertPos, iface.c_str(), chain.c_str()),StringPrintf("-I bw_OUTPUT %d -o %s -j %s", ruleInsertPos, iface.c_str(),chain.c_str()),StringPrintf("-A bw_FORWARD -i %s -j %s", iface.c_str(), chain.c_str()),StringPrintf("-A bw_FORWARD -o %s -j %s", iface.c_str(), chain.c_str()),StringPrintf("-A %s -m quota2 ! --quota %" PRId64 " --name %s -j REJECT", chain.c_str(),maxBytes, cost.c_str()),"COMMIT\n",};if (iptablesRestoreFunction(V4V6, Join(cmds, "\n"), nullptr) != 0) {ALOGE("Failed set quota rule");removeInterfaceQuota(iface);return -EREMOTEIO;}mQuotaIfaces[iface] = QuotaInfo{maxBytes, 0};return 0;
}

netd 向上提供此接口，内部除了ndc模块通过控制台可以调用，没有自动关注网络状态，调用setInterfaceQuota的地方：

binder::Status NetdNativeService::bandwidthSetInterfaceQuota(const std::string& ifName,int64_t bytes) {NETD_LOCKING_RPC(gCtls->bandwidthCtrl.lock, PERM_NETWORK_STACK, PERM_MAINLINE_NETWORK_STACK);int res = gCtls->bandwidthCtrl.setInterfaceQuota(ifName, bytes);return statusFromErrcode(res);
}

继续在android代码中搜索：
framework的NetworkManagementService.java中调用
mNetdService.bandwidthSetInterfaceQuota

    public void setInterfaceQuota(String iface, long quotaBytes) {NetworkStack.checkNetworkStackPermission(mContext);synchronized (mQuotaLock) {if (mActiveQuotas.containsKey(iface)) {throw new IllegalStateException("iface " + iface + " already has quota");}try {// TODO: support quota shared across interfacesmNetdService.bandwidthSetInterfaceQuota(iface, quotaBytes);mActiveQuotas.put(iface, quotaBytes);} catch (RemoteException | ServiceSpecificException e) {throw new IllegalStateException(e);}synchronized (mTetheringStatsProviders) {for (ITetheringStatsProvider provider : mTetheringStatsProviders.keySet()) {try {provider.setInterfaceQuota(iface, quotaBytes);} catch (RemoteException e) {Log.e(TAG, "Problem setting tethering data limit on provider " +mTetheringStatsProviders.get(provider) + ": " + e);}}}}}

调用序列是：
NatworkManagementService:
NatworkManagementService.systemReady->prepareNativeDaemon()->setInterfaceQuota->mNetdService.bandwidthSetInterfaceQuota

下一步分析数据业务开关后，是什么流程触发NatworkManagementService的setInterfaceQuota和removeInterfaceQuota的调用。

参考链接：
Android系统中iptables的应用（二）BandwidthController https://blog.csdn.net/xiaokeweng/article/details/48810049