From nobody Fri Dec 26 23:25:34 2025 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.86.151]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1801714281 for ; Fri, 29 Dec 2023 20:55:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ACULAB.COM Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=aculab.com Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-29-Sipm66Z9Nfy_PnSYO0hsLg-1; Fri, 29 Dec 2023 20:55:05 +0000 X-MC-Unique: Sipm66Z9Nfy_PnSYO0hsLg-1 Received: from AcuMS.Aculab.com (10.202.163.4) by AcuMS.aculab.com (10.202.163.4) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Fri, 29 Dec 2023 20:54:47 +0000 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Fri, 29 Dec 2023 20:54:47 +0000 From: David Laight To: "'linux-kernel@vger.kernel.org'" , "'peterz@infradead.org'" , "'longman@redhat.com'" CC: "'mingo@redhat.com'" , "'will@kernel.org'" , "'boqun.feng@gmail.com'" , "'Linus Torvalds'" , "'xinhui.pan@linux.vnet.ibm.com'" , "'virtualization@lists.linux-foundation.org'" , 'Zeng Heng' Subject: RE: [PATCH next 2/5] locking/osq_lock: Avoid dirtying the local cpu's 'node' in the osq_lock() fast path. Thread-Topic: [PATCH next 2/5] locking/osq_lock: Avoid dirtying the local cpu's 'node' in the osq_lock() fast path. Thread-Index: Ado6mUd9raebt4vmQzObd4YJJW0D/g== Date: Fri, 29 Dec 2023 20:54:47 +0000 Message-ID: <6eacbdbd1e0f489783439af512dc8325@AcuMS.aculab.com> References: <73a4b31c9c874081baabad9e5f2e5204@AcuMS.aculab.com> In-Reply-To: <73a4b31c9c874081baabad9e5f2e5204@AcuMS.aculab.com> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" osq_lock() starts by setting node->next to NULL and node->locked to 0. Careful analysis shows that node->next is always NULL on entry. node->locked is set non-zero by another cpu to force a wakeup. This can only happen after the 'prev->next =3D node' assignment, so locked can be set to zero just before that (along with the assignment to node->prev). Only initialise node->cpu once, after that use its value instead of smp_processor_id() - which is probably a real function call. Should reduce cache-line bouncing a little. Signed-off-by: David Laight --- kernel/locking/osq_lock.c | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index d414eef4bec6..55f5db896c02 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -51,7 +51,7 @@ osq_wait_next(struct optimistic_spin_queue *lock, struct optimistic_spin_node *prev) { struct optimistic_spin_node *next =3D NULL; - int curr =3D encode_cpu(smp_processor_id()); + int curr =3D node->cpu; int old; =20 /* @@ -98,12 +98,10 @@ bool osq_lock(struct optimistic_spin_queue *lock) { struct optimistic_spin_node *node =3D this_cpu_ptr(&osq_node); struct optimistic_spin_node *prev, *next; - int curr =3D encode_cpu(smp_processor_id()); int old; =20 - node->locked =3D 0; - node->next =3D NULL; - node->cpu =3D curr; + if (unlikely(node->cpu =3D=3D OSQ_UNLOCKED_VAL)) + node->cpu =3D encode_cpu(smp_processor_id()); =20 /* * We need both ACQUIRE (pairs with corresponding RELEASE in @@ -111,12 +109,13 @@ bool osq_lock(struct optimistic_spin_queue *lock) * the node fields we just initialised) semantics when updating * the lock tail. */ - old =3D atomic_xchg(&lock->tail, curr); + old =3D atomic_xchg(&lock->tail, node->cpu); if (old =3D=3D OSQ_UNLOCKED_VAL) return true; =20 prev =3D decode_cpu(old); node->prev =3D prev; + node->locked =3D 0; =20 /* * osq_lock() unqueue @@ -214,7 +213,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) void osq_unlock(struct optimistic_spin_queue *lock) { struct optimistic_spin_node *node, *next; - int curr =3D encode_cpu(smp_processor_id()); + int curr =3D raw_cpu_read(osq_node.cpu); =20 /* * Fast path for the uncontended case. --=20 2.17.1 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)