Re: [PATCH v5 0/4] Combine perf and bpf for fast eval of hw breakpoint conditions

[Alexei Starovoitov]

On Mon, Feb 12, 2024 at 8:37 AM Kyle Huey <me@xxxxxxxxxxxx> wrote:
>
> On Sun, Jan 21, 2024 at 10:25 PM Kyle Huey <me@xxxxxxxxxxxx> 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@xxxxxxxxxxxx/
> >
> > 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@xxxxxxxxxxxx/
> >
> > 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@xxxxxxxxxxxx/
> >
> > 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?

The changes look fine, but since they change perf side we need
perf maintainer's ack-s before we can land the patches.
And none of them were cc-ed.
So please resend the whole set and cc
PERFORMANCE EVENTS SUBSYSTEM
M:      Peter Zijlstra <peterz@xxxxxxxxxxxxx>
M:      Ingo Molnar <mingo@xxxxxxxxxx>
M:      Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
M:      Namhyung Kim <namhyung@xxxxxxxxxx>