> On Feb 23, 2021, at 6:34 AM, Marco Elver <elver@xxxxxxxxxx> wrote: > > 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. Unless I missed some machinations, which is entirely possible, you can’t call force_sig_info() from NMI context. Not only am I not convinced that the core signal code is NMI safe, but at least x86 can’t correctly deliver signals on NMI return. You probably need an IPI-to-self. > > 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). This would be doable much more simply with an API to set a breakpoint. All the machinery exists except the actual user API. > [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. Cute. > > 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. I’m skeptical. Propagating a hardware breakpoint to all threads involves IPIs and horribly slow writes to DR1 (or 2, 3, or 4) and DR7. A TLB flush can be accelerated using paravirt or hypothetical future hardware. Or real live hardware on ARM64. (The hypothetical future hardware is almost present on Zen 3. A bit of work is needed on the hardware end to make it useful.) > > 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 >
