[lttng-dev] [PATCH 00/11] Add support for TSAN to liburcu

[Ondřej Surý]

> On 26. 5. 2023, at 8:08, Dmitry Vyukov <dvyukov at google.com> wrote:
> 
> On Fri, 26 May 2023 at 07:33, Ondřej Surý <ondrej at sury.org> wrote:
>> 
>> A little bit related question/problem - some of our system test jobs got significantly slower (3x-5x) with **clang-16** TSAN after we integrated liburcu to BIND 9. The GCC (13.1.1) TSAN doesn’t exhibit this behavior.
>> 
>> The liburcu integration in BIND 9 is very light at the moment - we use liburcu for qptrie and there’s some usage of wfcqueue for queuing jobs(callbacks) between threads.
>> 
>> We were not able to put a finger on it yet, but saying this aloud might help diagnose this.
> 
> Interesting. Clang 16 got a completely new tsan runtime
> implementation, which is generally ~2x faster and consumes less
> memory.
> However, of course, it's not possible to optimize for all gazillion of
> programs in the world. I suspect librcu tests are quite extreme in
> terms of synchronization.
> 
> If you collect perf profiles for both, I can look for some low hanging fruit.
> Or is there a reasonable way for me to reproduce?

Looks like it got all resolved by using the latest liburcu patch set with full
TSAN support. I haven't realized that our CI still runs with whatever liburcu
Debian bullseye have. Sorry for the confusion.

And I can no longer reproduce this locally.

FTR here are some numbers from my local system (13th Gen Intel(R) Core(TM) i9-13900K)
running TSAN-enabled patched userspace-rcu[1], libuv 1.45.0 and OpenSSL 3.0.8:

gcc-13 baseline

rpz:            passed in 198.71s (0:03:18)
rpzrecurse:     passed in 310.62s (0:05:10)
rrsetorder:     passed in 200.40s (0:03:20)

gcc-13 TSAN

rpz:            passed in 305.64s (0:05:05)
rpzrecurse:     passed in 427.73s (0:07:07)
rrsetorder:     passed in 260.65s (0:04:20)

clang-16 TSAN

rpz:            passed in 290.05s (0:04:50)
rpzrecurse:     passed in 468.53s (0:07:48)
rrsetorder:     passed in 258.97s (0:04:18)

This looks quite reasonable to me.

1. git fetch https://review.lttng.org/userspace-rcu refs/changes/58/10058/1 && git checkout FETCH_HEAD

Ondrej
--
Ondřej Surý (He/Him)
ondrej at sury.org