1. 启动进程init log模块
init启动过程中, logcat模块还没有启动，日志只能输出到终端，或kernel日志系统， 输出到终端会影响启动速率，所以init启动过程日志是输出到kernel日志系统的。
从下面的代码片段，知道启动过程日志是通过LOG接口输入到dev/kmsg文件的。
**
system/core/init/init.cpp
994 // Now that tmpfs is mounted on /dev and we have /dev/kmsg, we can actually
995 // talk to the outside world...
996 InitKernelLogging(argv);
997
998 LOG(INFO) << "init first stage started!";
999
1000 if (!DoFirstStageMount()) {
1001 LOG(ERROR) << "Failed to mount required partitions early ...";
1002 panic();
1003 }
1004
system/core/init/log.cpp
26 void InitKernelLogging(char* argv[]) {
27 // Make stdin/stdout/stderr all point to /dev/null.
28 int fd = open("/sys/fs/selinux/null", O_RDWR);
29 if (fd == -1) {
30 int saved_errno = errno;
31 android::base::InitLogging(argv, &android::base::KernelLogger);
32 errno = saved_errno;
33 PLOG(FATAL) << "Couldn't open /sys/fs/selinux/null";
34 }
35 dup2(fd, 0);
36 dup2(fd, 1);
37 dup2(fd, 2);
38 if (fd > 2) close(fd);
39
40 android::base::InitLogging(argv, &android::base::KernelLogger);
41 }
system/core/base/logging.cpp
148 #if defined(__linux__)
149 void KernelLogger(android::base::LogId, android::base::LogSeverity severity,
150 const char* tag, const char*, unsigned int, const char* msg) {
151 // clang-format off
152 static constexpr int kLogSeverityToKernelLogLevel[] = {
153 [android::base::VERBOSE] = 7, // KERN_DEBUG (there is no verbose kernel log
154 // level)
155 [android::base::DEBUG] = 7, // KERN_DEBUG
156 [android::base::INFO] = 6, // KERN_INFO
157 [android::base::WARNING] = 4, // KERN_WARNING
158 [android::base::ERROR] = 3, // KERN_ERROR
159 [android::base::FATAL_WITHOUT_ABORT] = 2, // KERN_CRIT
160 [android::base::FATAL] = 2, // KERN_CRIT
161 };
162 // clang-format on
163 static_assert(arraysize(kLogSeverityToKernelLogLevel) == android::base::FATAL + 1,
164 "Mismatch in size of kLogSeverityToKernelLogLevel and values in LogSeverity");
165
166 static int klog_fd = TEMP_FAILURE_RETRY(open("/dev/kmsg", O_WRONLY | O_CLOEXEC));
167 if (klog_fd == -1) return;
168
169 int level = kLogSeverityToKernelLogLevel[severity];
170
171 // The kernel's printk buffer is only 1024 bytes.
172 // TODO: should we automatically break up long lines into multiple lines?
173 // Or we could log but with something like "..." at the end?
174 char buf[1024];
175 size_t size = snprintf(buf, sizeof(buf), "<%d>%s: %s\n", level, tag, msg);
176 if (size > sizeof(buf)) {
177 size = snprintf(buf, sizeof(buf), "<%d>%s: %zu-byte message too long for printk\n",
178 level, tag, size);
179 }
180
181 iovec iov[1];
182 iov[0].iov_base = buf;
183 iov[0].iov_len = size;
184 TEMP_FAILURE_RETRY(writev(klog_fd, iov, 1));
185 }
186 #endif
2. 打开启动过程log
经过上面问题，init启动过程日志主要是输出到/dev/kmsg, 如果查不到日志，可能日志被刷掉，或者受限kernel日志输出频率，被丢弃掉， 所以增加kernel log缓存和输出频率限制来查看日志，内核对性能要求比较高，这些也是临时修改。
**
diff --git a/bigfish/sdk/source/kernel/linux-4.9.y/include/linux/ratelimit.h b/bigfish/sdk/source/kernel/linux-4.9.y/include/linux/ratelimit.h
index 57c9e0622..d8cc7dba9 100644
--- a/bigfish/sdk/source/kernel/linux-4.9.y/include/linux/ratelimit.h
+++ b/bigfish/sdk/source/kernel/linux-4.9.y/include/linux/ratelimit.h
@@ -6,7 +6,7 @@
#include <linux/spinlock.h>
#define DEFAULT_RATELIMIT_INTERVAL (5 * HZ)
-#define DEFAULT_RATELIMIT_BURST 10
+#define DEFAULT_RATELIMIT_BURST 100000
/* issue num suppressed message on exit */
#define RATELIMIT_MSG_ON_RELEASE BIT(0)
diff --git a/bigfish/sdk/source/kernel/linux-4.9.y/kernel/printk/printk.c b/bigfish/sdk/source/kernel/linux-4.9.y/kernel/printk/printk.c
index 20fc294fb..9701e0f5a 100644
--- a/bigfish/sdk/source/kernel/linux-4.9.y/kernel/printk/printk.c
+++ b/bigfish/sdk/source/kernel/linux-4.9.y/kernel/printk/printk.c
@@ -388,7 +388,7 @@ static u32 clear_idx;
/* record buffer */
#define LOG_ALIGN __alignof__(struct printk_log)
-#define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
+#define __LOG_BUF_LEN ( 4*1024 *1024 )
static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;
通过 adb shell cat /proc/kmsg > kernel.log输出的kernel.log文件中搜索查看
作者：阿拉贡居民
链接：www.jianshu.com/p/84bda2be2…
来源：简书
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。
方法二
1. if (!(devkmsg_log & DEVKMSG_LOG_MASK_ON)) {
1. - if (!___ratelimit(&user->rs, current->comm))
1. - return ret;
1. + //if (!___ratelimit(&user->rs, current->comm))
1. + //return ret;
1. }
1. buf = kmalloc(len+1, GFP_KERNEL);
