From d637413f3f05b41f678f8004225b33b62274183f Mon Sep 17 00:00:00 2001 From: Jake Moilanen Date: Sun, 1 May 2005 08:58:47 -0700 Subject: [PATCH] [PATCH] ppc64: reverse prediction on spinlock busy loop code On our raw spinlocks, we currently have an attempt at the lock, and if we do not get it we enter a spin loop. This spinloop will likely continue for awhile, and we pridict likely. Shouldn't we predict that we will get out of the loop so our next instructions are already prefetched. Even when we miss because the lock is still held, it won't matter since we are waiting anyways. I did a couple quick benchmarks, but the results are inconclusive. 16-way 690 running specjbb with original code # ./specjbb 3000 16 1 1 19 30 120 ... Valid run, Score is 59282 16-way 690 running specjbb with unlikely code # ./specjbb 3000 16 1 1 19 30 120 ... Valid run, Score is 59541 I saw a smaller increase on a JS20 (~1.6%) JS20 specjbb w/ original code # ./specjbb 400 2 1 1 19 30 120 ... Valid run, Score is 20460 JS20 specjbb w/ unlikely code # ./specjbb 400 2 1 1 19 30 120 ... Valid run, Score is 20803 Anton said: Mispredicting the spinlock busy loop also means we slow down the rate at which we do the loads which can be good for heavily contended locks. Note: There are some gcc issues with our default build and branch prediction, but a CONFIG_POWER4_ONLY build should emit them correctly. I'm working with Alan Modra on it now. Signed-off-by: Jake Moilanen Signed-off-by: Anton Blanchard Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/asm-ppc64/spinlock.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/include/asm-ppc64/spinlock.h b/include/asm-ppc64/spinlock.h index a9b2a1162c..acd11564dd 100644 --- a/include/asm-ppc64/spinlock.h +++ b/include/asm-ppc64/spinlock.h @@ -110,7 +110,7 @@ static void __inline__ _raw_spin_lock(spinlock_t *lock) HMT_low(); if (SHARED_PROCESSOR) __spin_yield(lock); - } while (likely(lock->lock != 0)); + } while (unlikely(lock->lock != 0)); HMT_medium(); } } @@ -128,7 +128,7 @@ static void __inline__ _raw_spin_lock_flags(spinlock_t *lock, unsigned long flag HMT_low(); if (SHARED_PROCESSOR) __spin_yield(lock); - } while (likely(lock->lock != 0)); + } while (unlikely(lock->lock != 0)); HMT_medium(); local_irq_restore(flags_dis); } @@ -194,7 +194,7 @@ static void __inline__ _raw_read_lock(rwlock_t *rw) HMT_low(); if (SHARED_PROCESSOR) __rw_yield(rw); - } while (likely(rw->lock < 0)); + } while (unlikely(rw->lock < 0)); HMT_medium(); } } @@ -251,7 +251,7 @@ static void __inline__ _raw_write_lock(rwlock_t *rw) HMT_low(); if (SHARED_PROCESSOR) __rw_yield(rw); - } while (likely(rw->lock != 0)); + } while (unlikely(rw->lock != 0)); HMT_medium(); } } -- 2.39.5