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.
