* Kyle Huey <me@xxxxxxxxxxxx> wrote: > Peter, Ingo, could you take a look at this? > > ---- > > 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. I suppose this optimization makes sense. Patch quality still needs to be improved though - see my review comments. Thanks, Ingo
