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>
