The logger meets linux-kernel

Toward the end of December, LWN looked at the new push to move various subsystems specific to Android kernels into the mainline. There seems to be broad agreement that merging this code makes sense, but that agreement becomes rather less clear once the discussion moves to the merging of specific subsystems. Tim Bird's request for comments on the Android "logger" mechanism shows that, even with a relatively simple piece of code, there is still a lot of room for disagreement and problems can turn out to be larger than expected.

The logger is a simple device designed to collect log data from applications and make that data available to developers and the central logging system. It was designed around a number of goals, most of which are driven by the untrusted nature of Android applications: the system wants to allow those applications to send messages to the log in a reliable and trustworthy manner. So applications should not be able to fake who they are, consume too much memory with log data, or spam the log at the expense of data from the kernel or other applications. Writing to the log is also meant to be a fast operation; lots of log data is generated, but little of it is read, so the write operation is the one that should be the fastest.

The driver implements a handful of logging "devices" for different streams; they are known as "main," "events," "radio," and "minor." These devices are hardwired into the code; there is no way to change the set of devices at run time. Each device is implemented as a simple ring buffer in kernel space; writing to the device adds a message to the buffer (annotated with timestamp and process ID information) while any number of readers can pull data out of the buffer. Each log has 256KB of memory dedicated to it; that size, too, is wired into the code. There is a small set of ioctl() operations provided to get the size of the log or the amount of unread logging data stored there, or to flush all data from the log.

The first question to be raised was: why is this facility needed at all? The kernel already has a mechanism for letting user space add entries to the log stream. In mainline kernels, that is done by writing to /dev/kmsg; that device takes the data given to it and passes it straight to printk(). There are a few reasons why Android would not want to use this interface: it does not allow for the separation of logging streams (and, thus, is limited to the root account), it does not add process ID information, and printk() is quite a bit more expensive than simply copying the log data into kernel memory. While adding a duplicate logging infrastructure is obviously problematic, a case could be made that something like logger should be merged and used as a replacement for /dev/kmsg.

That said, "something like logger" need not be the logger itself. If nothing else, it is hard to imagine the current code being merged without corrections for the most obvious problems: the hardwired log streams, for example. Logger is entirely unaware of namespaces, making it unsuited to environments where different process groups may want their own logging streams. It also does not really succeed in keeping processes from spamming the logs and overwriting data from other processes; each of the four streams is isolated from the others, but applications still end up logging to the same streams.

There is also the question of whether the user-space API is the right one. Adding more ioctl() calls to the kernel is never a popular thing to do. Some participants have suggested that an entirely user-space-based logging daemon would work just as well. But a user-space solution has some disadvantages: it would not be able to provide trusted process ID information, and it would likely be slower. Communicating log data to a user-space daemon and adding information like time stamps would involve more system calls than simply logging the data through the kernel. It would also be hard to merge kernel and user-space log data (and, in particular, to ensure that the ordering between events is correct) with a user-space scheme.

Neil Brown suggested that a filesystem interface could be used instead of logger's char device ABI. A logging stream could be established simply by creating a file within that filesystem; regular file permissions could then be used to control access to the streams. The ioctl() calls could be replaced with regular filesystem calls; flushing a log would be done with truncate(), for example. A filesystem-based logger would, clearly, involve moving to a different user-space interface, but it seems like it should be able to satisfy the use case that led to the creation of logger with a more Unix-like ABI.

Android developer Brian Swetland has indicated that there might be interest in trying out an alternative logging implementation, but it would have to meet their needs and it's not something that they would be interested in creating themselves:

There is another point to consider as well: the longstanding interest in adding a more structured logging interface to the kernel seems to be getting stronger. Adding a new logging mechanism that doesn't address the concerns of translators, documentation writers, and people maintaining automated network management systems is going to be a hard sell. Kay Sievers, one of the developers behind "The Journal," has a plan of his own for logging; it involves adding more structured information to the existing message stream. He thinks there is no need to add a new, incompatible logging mechanism; he is also strongly opposed to the idea of separating message streams as logger does. That separation, he says, just creates problems when messages from multiple sources need to be merged back together in the correct order.

Brian actually seems receptive to some of these ideas, but has expressed concerns around rate-limiting and controlling access to log data. Lennart Poettering believes these needs can be met by the Journal with a little help from control groups. It seems possible that something could be done to make both groups happy - though the code to actually do so has not been posted.

As of this writing, that is where the discussion stands. On the surface, it looks like another piece of Android technology has gotten tangled up in unrelated requirements from various kernel developers, with the result that it will sink like its predecessors. But there is the potential for a number of possible solutions here. One, of course, would be to simply merge a moderately cleaned-up logger for Android's use and be done with it. Another would be to, as Neil suggested, focus on Android's user-space ABI for logging. If that ABI were formalized in a way that would allow multiple underlying implementations, the mainline could merge something other than logger and still be that much closer to being able to run Android's user space.

Perhaps the most intriguing outcome, though, would come if developers from Android, the Journal, and others with an interest in structured logging could get together and agree on what the next generation of logging should look like. Their solution would be likely to start with a critical mass of users from the outset, improving its chances of being merged and adopted by others. Given the number of structured logging efforts that have floundered in the past, getting a widely-accepted solution in place would be a major accomplishment.

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