On Thu, Oct 17, 2019 at 4:13 PM Marco Elver <elver@xxxxxxxxxx> wrote: > > Kernel Concurrency Sanitizer (KCSAN) is a dynamic data-race detector for > kernel space. KCSAN is a sampling watchpoint-based data-race detector. > See the included Documentation/dev-tools/kcsan.rst for more details. I think there is some significant potential for improving performance. Currently we have __tsan_read8 do 2 function calls, push/pop, the second call is on unpredicted slow path. Then __kcsan_check_watchpoint and __kcsan_setup_watchpoint do full load of spills and lots of loads and checks that are not strictly necessary or can be avoided. Additionally __kcsan_setup_watchpoint calls non-inlined kcsan_is_atomic. I think we need to try to structure it around the fast path as follows: __tsan_read8 does no function calls and no spills on fast path for both checking existing watchpoints and checking if a new watchpoint need to be setup. If it discovers a race with existing watchpoint or needs to setup a new one, that should be non-inlined tail calls to the corresponding slow paths. In particular, global enable/disable can be replaced with occupying/freeing all watchpoints. Per cpu disabled check should be removed from fast path somehow, it's only used around debugging checks or during reporting. There should be a way to check it on a slower path. user_access_save should be removed from fast path, we needed it only if we setup a watchpoint. But I am not sure why we need it at all, we should not be reading any user addresses. should_watch should be restructured to decrement kcsan_skip first, if it hits zero (with unlikely hint), we go to slow path. The slow path resets kcsan_skip to something random. The comment mentions prandom_u32 is too expensive, do I understand it correctly that you tried to call it on the fast path? I would expect it is fine for slow path and will give us better randomness. At this point we should return from __tsan_read8. To measure performance we could either do some synthetic in-kernel benchmarks (e.g. writing something to the debugfs file, which will do a number of memory accesses in a loop). Or you may try these user-space benchmarks: https://github.com/google/sanitizers/blob/master/address-sanitizer/kernel_buildbot/slave/bench_readv.c https://github.com/google/sanitizers/blob/master/address-sanitizer/kernel_buildbot/slave/bench_pipes.c
