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

[Marco Elver]

On Tue, 23 Feb 2021 at 15:34, 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.
>
> 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

Note that we're currently pondering fork + exec, and suggestions would
be appreciated. We think we'll need some restrictions, like Peter
proposed here: here:
https://lore.kernel.org/lkml/YBvj6eJR%2FDY2TsEB@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

We think what we want is to inherit the events to children only if
cloned with CLONE_SIGHAND. If there's space for a 'inherit_mask' in
perf_event_attr, that'd be most flexible, but perhaps we do not have
the space.

Thanks,
-- Marco

>
>  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
>