The perf subsystem today unifies various tracing and monitoring features, from both software and hardware. One benefit of the perf subsystem is automatically inheriting events to child tasks, which enables process-wide events monitoring with low overheads. By default perf events are non-intrusive, not affecting behaviour of the tasks being monitored. For certain use-cases, however, it makes sense to leverage the generality of the perf events subsystem and optionally allow the tasks being monitored to receive signals on events they are interested in. This patch series adds the option to synchronously signal user space on events. The discussion at [1] led to the changes proposed in this series. The approach taken in patch 3/4 to use 'event_limit' to trigger the signal was kindly suggested by Peter Zijlstra in [2]. [1] https://lore.kernel.org/lkml/CACT4Y+YPrXGw+AtESxAgPyZ84TYkNZdP0xpocX2jwVAbZD=-XQ@xxxxxxxxxxxxxx/ [2] https://lore.kernel.org/lkml/YBv3rAT566k+6zjg@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ Motivation and example uses: 1. Our immediate motivation is low-overhead sampling-based race detection for user-space [3]. By using perf_event_open() at process initialization, we can create hardware breakpoint/watchpoint events that are propagated automatically to all threads in a process. As far as we are aware, today no existing kernel facility (such as ptrace) allows us to set up process-wide watchpoints with minimal overheads (that are comparable to mprotect() of whole pages). [3] https://llvm.org/devmtg/2020-09/slides/Morehouse-GWP-Tsan.pdf 2. Other low-overhead error detectors that rely on detecting accesses to certain memory locations or code, process-wide and also only in a specific set of subtasks or threads. Other example use-cases we found potentially interesting: 3. Code hot patching without full stop-the-world. Specifically, by setting a code breakpoint to entry to the patched routine, then send signals to threads and check that they are not in the routine, but without stopping them further. If any of the threads will enter the routine, it will receive SIGTRAP and pause. 4. Safepoints without mprotect(). Some Java implementations use "load from a known memory location" as a safepoint. When threads need to be stopped, the page containing the location is mprotect()ed and threads get a signal. This can be replaced with a watchpoint, which does not require a whole page nor DTLB shootdowns. 5. Tracking data flow globally. 6. Threads receiving signals on performance events to throttle/unthrottle themselves. Marco Elver (4): perf/core: Apply PERF_EVENT_IOC_MODIFY_ATTRIBUTES to children signal: Introduce TRAP_PERF si_code and si_perf to siginfo perf/core: Add support for SIGTRAP on perf events perf/core: Add breakpoint information to siginfo on SIGTRAP arch/m68k/kernel/signal.c | 3 ++ arch/x86/kernel/signal_compat.c | 5 ++- fs/signalfd.c | 4 +++ include/linux/compat.h | 2 ++ include/linux/signal.h | 1 + include/uapi/asm-generic/siginfo.h | 6 +++- include/uapi/linux/perf_event.h | 3 +- include/uapi/linux/signalfd.h | 4 ++- kernel/events/core.c | 54 +++++++++++++++++++++++++++++- kernel/signal.c | 11 ++++++ 10 files changed, 88 insertions(+), 5 deletions(-) -- 2.30.0.617.g56c4b15f3c-goog
