Messages in this thread | | | Date | Wed, 5 Jan 2022 22:34:31 -0500 | Subject | Re: [PATCH] locking/local_lock: Make the empty local_lock_*() function a macro. | From | Waiman Long <> |
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On 1/5/22 15:26, Sebastian Andrzej Siewior wrote: > It has been said that local_lock() does not add any overhead compared to > preempt_disable() in a !LOCKDEP configuration. A microbenchmark showed > an unexpected result which can be reduced to the fact that local_lock() > was not entirely optimized away. > In the !LOCKDEP configuration local_lock_acquire() is an empty static > inline function. On x86 the this_cpu_ptr() argument of that function is > fully evaluated leading to an additional mov+add instructions which are > not needed and not used. > > Replace the static inline function with a macro. The typecheck() macro > ensures that the argument is of proper type while the resulting > disassembly shows no traces of this_cpu_ptr(). > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > On -rc8, size says: > | text data bss dec filename > | 19656718 8681015 3764440 32102173 vmlinux.old > | 19656218 8681015 3764440 32101673 vmlinux.new > > Which is -500 text, not much but still. > > include/linux/local_lock_internal.h | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h > index 975e33b793a77..6d635e8306d64 100644 > --- a/include/linux/local_lock_internal.h > +++ b/include/linux/local_lock_internal.h > @@ -44,9 +44,9 @@ static inline void local_lock_debug_init(local_lock_t *l) > } > #else /* CONFIG_DEBUG_LOCK_ALLOC */ > # define LOCAL_LOCK_DEBUG_INIT(lockname) > -static inline void local_lock_acquire(local_lock_t *l) { } > -static inline void local_lock_release(local_lock_t *l) { } > -static inline void local_lock_debug_init(local_lock_t *l) { } > +# define local_lock_acquire(__ll) do { typecheck(local_lock_t *, __ll); } while (0) > +# define local_lock_release(__ll) do { typecheck(local_lock_t *, __ll); } while (0) > +# define local_lock_debug_init(__ll) do { typecheck(local_lock_t *, __ll); } while (0) > #endif /* !CONFIG_DEBUG_LOCK_ALLOC */ > > #define INIT_LOCAL_LOCK(lockname) { LOCAL_LOCK_DEBUG_INIT(lockname) }
I try out this patch and it indeed helps to reduce the object size of functions that use local_lock(). However, the extra code isn't an additional mov+add.
Using folio_add_lru() as an example,
Without the patch:
466 local_lock(&lru_pvecs.lock); 0x00000000000032ee <+14>: mov $0x1,%edi 0x00000000000032f3 <+19>: callq 0x32f8 <folio_add_lru+24> 0x00000000000032f8 <+24>: callq 0x32fd <folio_add_lru+29>
With the patch:
466 local_lock(&lru_pvecs.lock); 0x00000000000032ae <+14>: mov $0x1,%edi 0x00000000000032b3 <+19>: callq 0x32b8 <folio_add_lru+24>
There is one less placeholder for tracing. Maybe it depends on the compiler and the exact config options.
Anyway,
Reviewed-by: Waiman Long <longman@redhat.com>
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