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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-485885bbb51sm27883762f8f.30.2026.09.07.01.41.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 07 Sep 2026 01:41:51 -0700 (PDT) From: David Laight To: Waiman Long , Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org, Linus Torvalds , Yafang Shao , Steven Rostedt Cc: David Laight Subject: [PATCH v4 next 2/9] locking/osq_lock: Save the cpu number for 'prev' not the node address Date: Mon, 7 Sep 2026 09:41:26 +0100 Message-Id: <20260907084133.3696-3-david.laight.linux@gmail.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20260907084133.3696-1-david.laight.linux@gmail.com> References: <20260907084133.3696-1-david.laight.linux@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The cpu number of node->prev is needed for both the vcpu_is_preempted() test and to update lock->tail. This saves reading the cache line for the other cpu's per-cpu data. The cpu member of optimistic_spin_node is no longer needed. Merges patches 2 and 3 from v3. Signed-off-by: David Laight --- kernel/locking/osq_lock.c | 33 ++++++++++++++------------------- 1 file changed, 14 insertions(+), 19 deletions(-) diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index b17aa704c449..01988d00c480 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -34,9 +34,9 @@ */ struct optimistic_spin_node { - struct optimistic_spin_node *next, *prev; + struct optimistic_spin_node *next; int locked; /* 1 if lock acquired */ - int cpu; /* encoded CPU # + 1 value */ + int prev; /* CPU number offset by 1 */ }; static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node); @@ -50,11 +50,6 @@ static inline int encode_cpu(int cpu_nr) return cpu_nr + 1; } -static inline int node_cpu(struct optimistic_spin_node *node) -{ - return node->cpu - 1; -} - static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) { int cpu_nr = encoded_cpu_val - 1; @@ -114,13 +109,12 @@ osq_wait_next(struct optimistic_spin_queue *lock, bool osq_lock(struct optimistic_spin_queue *lock) { struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); - struct optimistic_spin_node *prev, *next; + struct optimistic_spin_node *prev_ptr, *next; int curr = encode_cpu(smp_processor_id()); - int old; + int prev; node->locked = 0; node->next = NULL; - node->cpu = curr; /* * We need both ACQUIRE (pairs with corresponding RELEASE in @@ -128,11 +122,11 @@ bool osq_lock(struct optimistic_spin_queue *lock) * the node fields we just initialised) semantics when updating * the lock tail. */ - old = atomic_xchg(&lock->tail, curr); - if (old == OSQ_UNLOCKED_VAL) + prev = atomic_xchg(&lock->tail, curr); + if (prev == OSQ_UNLOCKED_VAL) return true; - prev = decode_cpu(old); + prev_ptr = decode_cpu(prev); node->prev = prev; /* @@ -147,7 +141,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) */ smp_wmb(); - WRITE_ONCE(prev->next, node); + WRITE_ONCE(prev_ptr->next, node); /* * Normally @prev is untouchable after the above store; because at that @@ -165,7 +159,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) * polling, be careful. */ if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() || - vcpu_is_preempted(node_cpu(node->prev)))) + vcpu_is_preempted(node->prev - 1))) return true; /* unqueue */ @@ -182,8 +176,8 @@ bool osq_lock(struct optimistic_spin_queue *lock) * cpu_relax() below implies a compiler barrier which would * prevent this comparison being optimized away. */ - if (data_race(prev->next) == node && - cmpxchg(&prev->next, node, NULL) == node) + if (data_race(prev_ptr->next) == node && + cmpxchg(&prev_ptr->next, node, NULL) == node) break; /* @@ -201,6 +195,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) * case its step-C will write us a new @node->prev pointer. */ prev = READ_ONCE(node->prev); + prev_ptr = decode_cpu(prev); } /* @@ -210,7 +205,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) * back to @prev. */ - next = osq_wait_next(lock, node, prev->cpu); + next = osq_wait_next(lock, node, prev); if (!next) return false; @@ -223,7 +218,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) */ WRITE_ONCE(next->prev, prev); - WRITE_ONCE(prev->next, next); + WRITE_ONCE(prev_ptr->next, next); return false; } -- 2.39.5