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

[Peter Zijlstra]

On Fri, Jan 31, 2025 at 04:47:06PM -0500, Waiman Long wrote:
> 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@xxxxxxxxxx>
> > ---
> >   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.

Perhaps put that in the changelog instead?

Its not like RT is this secret out of tree project :-)

Also, any quick clues as to what causes the extra lockdep overhead?
Initially I thought perhaps local-lock, but that should also cause
lockdep on !RT builds.