[v2,0/3] Combine perf and bpf for fast eval of hw breakpoint conditions

Message

Kyle Huey Dec. 7, 2023, 4:34 p.m. UTC

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. This appears to be a simple
oversight in the code. This patch set reorders the overflow handler
callback and the side effects of perf event overflow to allow an overflow
handler to suppress all side effects, changes bpf_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/[e-mail]/

Changes since v1:

Patch 1 was added so that a sample suppressed by this mechanism will also
not generate SIGTRAPs nor count against the event limit.

Patch 2 is v1's patch 1.

Patch 3 is v1's patch 2, and addresses a number of review comments about
the self test and adds testing for the behavior introduced by patch 1.

[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.