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Message-ID: <Z6w4UlCQa_g1OHlN@Mac.home> Date: Tue, 11 Feb 2025 21:57:38 -0800 From: Boqun Feng <boqun.feng@...il.com> To: Waiman Long <longman@...hat.com> Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org, Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>, kasan-dev@...glegroups.com Subject: Re: [PATCH v3 3/3] locking/lockdep: Disable KASAN instrumentation of lockdep.c [Cc KASAN] A Reviewed-by or Acked-by from KASAN would be nice, thanks! Regards, Boqun On Sun, Feb 09, 2025 at 11:26:12PM -0500, Waiman Long wrote: > Both KASAN and LOCKDEP are commonly enabled in building a debug kernel. > Each of them can significantly slow down the speed of a debug kernel. > Enabling KASAN instrumentation of the LOCKDEP code will further slow > thing down. > > Since LOCKDEP is a high overhead debugging tool, it will never get > enabled in a production kernel. The LOCKDEP code is also pretty mature > and is unlikely to get major changes. There is also a possibility of > recursion similar to KCSAN. > > To evaluate the performance impact of disabling KASAN instrumentation > of lockdep.c, the time to do a parallel build of the Linux defconfig > kernel was used as the benchmark. Two x86-64 systems (Skylake & Zen 2) > and an arm64 system were used as test beds. Two sets of non-RT and RT > kernels with similar configurations except mainly CONFIG_PREEMPT_RT > were used for evaulation. > > For the Skylake system: > > Kernel Run time Sys time > ------ -------- -------- > Non-debug kernel (baseline) 0m47.642s 4m19.811s > Debug kernel 2m11.108s (x2.8) 38m20.467s (x8.9) > Debug kernel (patched) 1m49.602s (x2.3) 31m28.501s (x7.3) > Debug kernel > (patched + mitigations=off) 1m30.988s (x1.9) 26m41.993s (x6.2) > > RT kernel (baseline) 0m54.871s 7m15.340s > RT debug kernel 6m07.151s (x6.7) 135m47.428s (x18.7) > RT debug kernel (patched) 3m42.434s (x4.1) 74m51.636s (x10.3) > RT debug kernel > (patched + mitigations=off) 2m40.383s (x2.9) 57m54.369s (x8.0) > > For the Zen 2 system: > > Kernel Run time Sys time > ------ -------- -------- > Non-debug kernel (baseline) 1m42.806s 39m48.714s > Debug kernel 4m04.524s (x2.4) 125m35.904s (x3.2) > Debug kernel (patched) 3m56.241s (x2.3) 127m22.378s (x3.2) > Debug kernel > (patched + mitigations=off) 2m38.157s (x1.5) 92m35.680s (x2.3) > > RT kernel (baseline) 1m51.500s 14m56.322s > RT debug kernel 16m04.962s (x8.7) 244m36.463s (x16.4) > RT debug kernel (patched) 9m09.073s (x4.9) 129m28.439s (x8.7) > RT debug kernel > (patched + mitigations=off) 3m31.662s (x1.9) 51m01.391s (x3.4) > > For the arm64 system: > > Kernel Run time Sys time > ------ -------- -------- > Non-debug kernel (baseline) 1m56.844s 8m47.150s > Debug kernel 3m54.774s (x2.0) 92m30.098s (x10.5) > Debug kernel (patched) 3m32.429s (x1.8) 77m40.779s (x8.8) > > RT kernel (baseline) 4m01.641s 18m16.777s > RT debug kernel 19m32.977s (x4.9) 304m23.965s (x16.7) > RT debug kernel (patched) 16m28.354s (x4.1) 234m18.149s (x12.8) > > Turning the mitigations off doesn't seems to have any noticeable impact > on the performance of the arm64 system. So the mitigation=off entries > aren't included. > > For the x86 CPUs, cpu mitigations has a much bigger impact on > performance, especially the RT debug kernel. The SRSO mitigation in > Zen 2 has an especially big impact on the debug kernel. It is also the > majority of the slowdown with mitigations on. It is because the patched > ret instruction slows down function returns. A lot of helper functions > that are normally compiled out or inlined may become real function > calls in the debug kernel. The KASAN instrumentation inserts a lot > of __asan_loadX*() and __kasan_check_read() function calls to memory > access portion of the code. The lockdep's __lock_acquire() function, > for instance, has 66 __asan_loadX*() and 6 __kasan_check_read() calls > added with KASAN instrumentation. Of course, the actual numbers may vary > depending on the compiler used and the exact version of the lockdep code. > > With the newly added rtmutex and lockdep lock events, the relevant > event counts for the test runs with the Skylake system were: > > Event type Debug kernel RT debug kernel > ---------- ------------ --------------- > lockdep_acquire 1,968,663,277 5,425,313,953 > rtlock_slowlock - 401,701,156 > rtmutex_slowlock - 139,672 > > The __lock_acquire() calls in the RT debug kernel are x2.8 times of the > non-RT debug kernel with the same workload. Since the __lock_acquire() > function is a big hitter in term of performance slowdown, this makes > the RT debug kernel much slower than the non-RT one. The average lock > nesting depth is likely to be higher in the RT debug kernel too leading > to longer execution time in the __lock_acquire() function. > > As the small advantage of enabling KASAN instrumentation to catch > potential memory access error in the lockdep debugging tool is probably > not worth the drawback of further slowing down a debug kernel, disable > KASAN instrumentation in the lockdep code to allow the debug kernels > to regain some performance back, especially for the RT debug kernels. > > Signed-off-by: Waiman Long <longman@...hat.com> > --- > kernel/locking/Makefile | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile > index 0db4093d17b8..a114949eeed5 100644 > --- a/kernel/locking/Makefile > +++ b/kernel/locking/Makefile > @@ -5,7 +5,8 @@ KCOV_INSTRUMENT := n > > obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o > > -# Avoid recursion lockdep -> sanitizer -> ... -> lockdep. > +# Avoid recursion lockdep -> sanitizer -> ... -> lockdep & improve performance. > +KASAN_SANITIZE_lockdep.o := n > KCSAN_SANITIZE_lockdep.o := n > > ifdef CONFIG_FUNCTION_TRACER > -- > 2.48.1 >
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