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New Articles in LWN About Kernel Space (Linux)

Submitted by Rianne Schestowitz on Thursday 22nd of April 2021 06:22:47 AM Filed under
Linux
  • Comparing SystemTap and bpftrace

    There are times when developers and system administrators need to diagnose problems in running code. The program to be examined can be a user-space process, the kernel, or both. Two of the major tools available on Linux to perform this sort of analysis are SystemTap and bpftrace. SystemTap has been available since 2005, while bpftrace is a more recent contender that, to some, may appear to have made SystemTap obsolete. However, SystemTap is still the preferred tool for some real-world use cases.

    Although dynamic instrumentation capabilities, in the form of KProbes, were added to Linux as early as 2004, the functionality was hard to use and not particularly well known. Sun released DTrace one year later, and soon that system became one of the highlights of Solaris. Naturally, Linux users started asking for something similar, and SystemTap quickly emerged as the most promising answer. But SystemTap was criticized as being difficult to get working, while DTrace on Solaris could be expected to simply work out of the box.

    While DTrace came with both kernel and user-space tracing capabilities, it wasn't until 2012 that Linux gained support for user-space tracing in the form of Uprobes. Around 2019, bpftrace gained significant traction, in part due to the general attention being paid to the various use cases for BPF. More recently, Oracle has been working on a re-implementation of DTrace, for Linux, based on the latest tracing facilities in the kernel, although, at this point, it may be too late for DTrace given the options that are already available in this space.

    The underlying kernel infrastructure used by both SystemTap and bpftrace is largely the same: KProbes, for dynamically tracing kernel functions, tracepoints for static kernel instrumentation, Uprobes for dynamic instrumentation of user-level functions, and user-level statically defined tracing (USDT) for static user-space instrumentation. Both systems allow instrumenting the kernel and user-space programs through a "script" in a high-level language that can be used to specify what needs to be probed and how.

  • NUMA-aware qspinlocks

    While some parts of the core kernel reached a relatively stable "done" state years ago, others never really seem to be finished. One of the latter variety is undoubtedly the kernel's implementation of spinlocks, which arbitrate access to data at the lowest levels of the kernel. Lock performance can have a significant effect on the performance of the system as a whole, so optimization work can pay back big dividends. Lest one think that this work is finally done, the NUMA-aware qspinlock patch set shows how some more performance can be squeezed out of the kernel's spinlock implementation.

    In its simplest form, a spinlock is a single word in memory, initially set to one. Any CPU wishing to acquire the lock will perform an atomic decrement-and-test operation; if the result is zero, the lock has been successfully taken. Otherwise the CPU will increment the value, then "spin" in tight loop until the operation succeeds. The kernel has long since left this sort of implementation behind, though, for a number of reasons, including performance. All those atomic operations on the lock word cause its cache line to be bounced around the system, slowing things considerably even if contention for the lock is light.

    The current "qspinlock" implementation is based on MCS locks, which implement a queue of CPUs waiting for the lock as a simple linked list. Normally, linked lists are just the sort of data structure that one wants to avoid when cache efficiency is a concern, but nobody ever has to traverse this list. Instead, each CPU will spin on its own entry in the list, and only reach into the next entry to release the lock. See this article for a more complete description, complete with cheesy diagrams, of how MCS locks work.

  • Seccomp user-space notification and signals

    The seccomp() mechanism allows the imposition of a filter program (expressed in "classic" BPF) that makes policy decisions on whether to allow each system call invoked by the target process. The user-space notification feature further allows those decisions to be deferred to another process. As this recent patch set from Sargun Dhillon shows, though, user-space notification still has some rough edges, especially when it comes to signals. This patch makes a simple change to try to address a rather complex problem brought to the fore by changes in the Go language's preemption model.
    Normally, seccomp() is used to implement a simple sort of attack-surface reduction, making much of the system-call space off limits for the affected process. User-space notification can be used to that end, but the objective there is often different: it allows a supervisor process to emulate system calls for the target process. An example might be a container manager that wishes to make mount() available inside a container, but with some strict limits on what can actually be mounted. User-space notification allows the (privileged) supervisor to actually perform the mount operations it approves of and return the results to the target process.

    While the supervisor is handling an intercepted system call, the target process will be blocked in the kernel, waiting for a response to come back. Should that process receive a signal, though, it will stop waiting and respond immediately to the signal; if the signal itself is not fatal, the result may well be the system call returning an EINTR error to the target process. The supervisor, instead, will not know about the signal until it tries to give the kernel its answer to the original notification; at that point, it will get an ENOENT error indicating that the notification is no longer alive.

    This sort of interruption can be inconvenient, especially if the supervisor has carried out some sort of long task on the target's behalf. If the signal does not kill the target process, it is likely that the same operation will be retried shortly, leading to extra work being done. Most of the time, though, non-fatal signals of this type are likely to be rare in programs running under seccomp() monitoring.

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