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From: Jiri Olsa <olsajiri@gmail.com>
Date: Fri, 19 Jan 2024 12:51:51 +0100
To: Kyle Huey <me@kylehuey.com>
Cc: Kyle Huey <khuey@kylehuey.com>, linux-kernel@vger.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Namhyung Kim <namhyung@kernel.org>, Marco Elver <elver@google.com>,
	Yonghong Song <yonghong.song@linux.dev>, Song Liu <song@kernel.org>,
	Robert O'Callahan <robert@ocallahan.org>, bpf@vger.kernel.org
Subject: Re: [PATCH v4 0/4] Combine perf and bpf for fast eval of hw
 breakpoint conditions
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References: <20240119001352.9396-1-khuey@kylehuey.com>
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In-Reply-To: <20240119001352.9396-1-khuey@kylehuey.com>

On Thu, Jan 18, 2024 at 04:13:47PM -0800, Kyle Huey 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/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.

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> 
> v2 of this patchset can be found at
> https://lore.kernel.org/linux-kernel/20231207163458.5554-1-khuey@kylehuey.com/
> 
> 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.
> 
>