Re: [PATCH] locking/lockdep: Disable KASAN instrumentation of lockdep.c

[Waiman Long <llong@redhat.com>]

On 1/31/25 11:50 AM, 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. As the small advantage of enabling KASAN
> instrumentation to catch potential memory access error is probably
> not worth the drawback of further slowing down a debug kernel, disable
> KASAN instrumentation to enable a debug kernel to gain a little bit of
> speed back.
>
> With a debug kernel with both LOCKDEP and KASAN enabled running on a
> 2-socket 144-thread system, the time to do a "make -j144" kernel build
> was 18m40.641s. After applying this patch, the parallel kernel build
> time was reduced to 17m35.136s. This is a reduction of about 66s (5.8%).
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/locking/Makefile | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
> index 0db4093d17b8..8a588b0227b1 100644
> --- a/kernel/locking/Makefile
> +++ b/kernel/locking/Makefile
> @@ -6,6 +6,7 @@ KCOV_INSTRUMENT		:= n
>   obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o
>   
>   # Avoid recursion lockdep -> sanitizer -> ... -> lockdep.
> +KASAN_SANITIZE_lockdep.o := n
>   KCSAN_SANITIZE_lockdep.o := n
>   
>   ifdef CONFIG_FUNCTION_TRACER

The rationale behind this patch is due to the fact that a similar 
configured PREEMPT_RT debug kernel is found to be about 3 times slower 
than the non-RT debug kernel. For the test same system, the parallel 
build runtime is 59m56.722s. After applying this patch, it is reduced to 
38m3.348s. Its more than 1/3 reduction is more than I would have 
expected. So the lockdep code is much more heavily used in a PREEMPT_RT 
debug kernel.

Cheers,
Longman