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SubjectRe: [PATCH RFC 0/4] Add support for synchronous signals on perf events
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> On Feb 23, 2021, at 6:34 AM, Marco Elver <elver@google.com> 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@mail.gmail.com/
> [2] https://lore.kernel.org/lkml/YBv3rAT566k+6zjg@hirez.programming.kicks-ass.net/
>
> 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
>

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