Re: [PATCH RFC 0/4] Add support for synchronous signals on perf events

[Andy Lutomirski]

> 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
>