Message-ID: <CAP045Apecy=G_Wmcw6TMjSDfa3TbkMfFVkzGDJ9xTVksCLkZ0w@mail.gmail.com>
Date: Mon, 12 Feb 2024 08:29:53 -0800
From: Kyle Huey <me@...ehuey.com>
To: Kyle Huey <khuey@...ehuey.com>, linux-kernel@...r.kernel.org, 
	Andrii Nakryiko <andrii.nakryiko@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	Namhyung Kim <namhyung@...nel.org>, Marco Elver <elver@...gle.com>, 
	Yonghong Song <yonghong.song@...ux.dev>
Cc: "Robert O'Callahan" <robert@...llahan.org>, bpf@...r.kernel.org
Subject: Re: [PATCH v5 0/4] Combine perf and bpf for fast eval of hw
 breakpoint conditions

On Sun, Jan 21, 2024 at 10:25 PM Kyle Huey <me@...ehuey.com> wrote:
>
> rr, a userspace record and replay debugger[0], replays asynchronous events
> such as signals and context switches by essentially[1] setting a breakpoint
> at the address where the asynchronous event was delivered during recording
> with a condition that the program state matches the state when the event
> was delivered.
>
> Currently, rr uses software breakpoints that trap (via ptrace) to the
> supervisor, and evaluates the condition from the supervisor. If the
> asynchronous event is delivered in a tight loop (thus requiring the
> breakpoint condition to be repeatedly evaluated) the overhead can be
> immense. A patch to rr that uses hardware breakpoints via perf events with
> an attached BPF program to reject breakpoint hits where the condition is
> not satisfied reduces rr's replay overhead by 94% on a pathological (but a
> real customer-provided, not contrived) rr trace.
>
> The only obstacle to this approach is that while the kernel allows a BPF
> program to suppress sample output when a perf event overflows it does not
> suppress signalling the perf event fd or sending the perf event's SIGTRAP.
> This patch set redesigns __perf_overflow_handler() and
> bpf_overflow_handler() so that the former invokes the latter directly when
> appropriate rather than through the generic overflow handler machinery,
> passes the return code of the BPF program back to __perf_overflow_handler()
> to allow it to decide whether to execute the regular overflow handler,
> reorders bpf_overflow_handler() and the side effects of perf event
> overflow, changes __perf_overflow_handler() to suppress those side effects
> if the BPF program returns zero, and adds a selftest.
>
> The previous version of this patchset can be found at
> https://lore.kernel.org/linux-kernel/20240119001352.9396-1-khuey@kylehueycom/
>
> Changes since v4:
>
> Patches 1, 2, 3, 4 added various Acked-by.
>
> Patch 4 addresses additional nits from Song.
>
> v3 of this patchset can be found at
> https://lore.kernel.org/linux-kernel/20231211045543.31741-1-khuey@kylehuey.com/
>
> Changes since v3:
>
> Patches 1, 2, 3 added various Acked-by.
>
> Patch 4 addresses Song's review comments by dropping signals_expected and the
> corresponding ASSERT_OKs, handling errors from signal(), and fixing multiline
> comment formatting.
>
> v2 of this patchset can be found at
> https://lore.kernel.org/linux-kernel/20231207163458.5554-1-khuey@kylehueycom/
>
> Changes since v2:
>
> Patches 1 and 2 were added from a suggestion by Namhyung Kim to refactor
> this code to implement this feature in a cleaner way. Patch 2 is separated
> for the benefit of the ARM arch maintainers.
>
> Patch 3 conceptually supercedes v2's patches 1 and 2, now with a cleaner
> implementation thanks to the earlier refactoring.
>
> Patch 4 is v2's patch 3, and addresses review comments about C++ style
> comments, getting a TRAP_PERF definition into the test, and unnecessary
> NULL checks.
>
> [0] https://rr-project.org/
> [1] Various optimizations exist to skip as much as execution as possible
> before setting a breakpoint, and to determine a set of program state that
> is practical to check and verify.

Since everyone seems to be satisfied with this now, can we get it into
bpf-next (or wherever) for 6.9?

- Kyle

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