From patchwork Sun Jun 15 12:47:02 2014 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Zijlstra X-Patchwork-Id: 4354511 Return-Path: X-Original-To: patchwork-kvm@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork1.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.19.201]) by patchwork1.web.kernel.org (Postfix) with ESMTP id 683C99F314 for ; Sun, 15 Jun 2014 13:17:51 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 79BD920212 for ; Sun, 15 Jun 2014 13:17:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6CE1C201FA for ; Sun, 15 Jun 2014 13:17:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751347AbaFONRq (ORCPT ); Sun, 15 Jun 2014 09:17:46 -0400 Received: from casper.infradead.org ([85.118.1.10]:41582 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751337AbaFONLo (ORCPT ); Sun, 15 Jun 2014 09:11:44 -0400 Received: from dhcp-077-248-225-117.chello.nl ([77.248.225.117] helo=laptop) by casper.infradead.org with esmtpsa (Exim 4.80.1 #2 (Red Hat Linux)) id 1WwADj-0003Uw-HU; Sun, 15 Jun 2014 13:11:39 +0000 Received: by laptop (Postfix, from userid 0) id 90ABC107F0745; Sun, 15 Jun 2014 15:11:36 +0200 (CEST) Message-Id: <20140615130153.483502389@chello.nl> User-Agent: quilt/0.61-1 Date: Sun, 15 Jun 2014 14:47:02 +0200 From: Peter Zijlstra To: Waiman.Long@hp.com, tglx@linutronix.de, mingo@kernel.org Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, xen-devel@lists.xenproject.org, kvm@vger.kernel.org, paolo.bonzini@gmail.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com, paulmck@linux.vnet.ibm.com, riel@redhat.com, torvalds@linux-foundation.org, raghavendra.kt@linux.vnet.ibm.com, david.vrabel@citrix.com, oleg@redhat.com, gleb@redhat.com, scott.norton@hp.com, chegu_vinod@hp.com, Peter Zijlstra Subject: [PATCH 05/11] qspinlock: Optimize for smaller NR_CPUS References: <20140615124657.264658593@chello.nl> MIME-Version: 1.0 Content-Disposition: inline; filename=peter_zijlstra-qspinlock-optimize_for_smaller_nr_cpus.patch Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org X-Spam-Status: No, score=-7.5 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_HI, RP_MATCHES_RCVD, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Peter Zijlstra When we allow for a max NR_CPUS < 2^14 we can optimize the pending wait-acquire and the xchg_tail() operations. By growing the pending bit to a byte, we reduce the tail to 16bit. This means we can use xchg16 for the tail part and do away with all the repeated compxchg() operations. This in turn allows us to unconditionally acquire; the locked state as observed by the wait loops cannot change. And because both locked and pending are now a full byte we can use simple stores for the state transition, obviating one atomic operation entirely. All this is horribly broken on Alpha pre EV56 (and any other arch that cannot do single-copy atomic byte stores). Signed-off-by: Peter Zijlstra --- include/asm-generic/qspinlock_types.h | 13 ++++ kernel/locking/qspinlock.c | 103 ++++++++++++++++++++++++++++++---- 2 files changed, 106 insertions(+), 10 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html --- a/include/asm-generic/qspinlock_types.h +++ b/include/asm-generic/qspinlock_types.h @@ -38,6 +38,14 @@ typedef struct qspinlock { /* * Bitfields in the atomic value: * + * When NR_CPUS < 16K + * 0- 7: locked byte + * 8: pending + * 9-15: not used + * 16-17: tail index + * 18-31: tail cpu (+1) + * + * When NR_CPUS >= 16K * 0- 7: locked byte * 8: pending * 9-10: tail index @@ -50,7 +58,11 @@ typedef struct qspinlock { #define _Q_LOCKED_MASK _Q_SET_MASK(LOCKED) #define _Q_PENDING_OFFSET (_Q_LOCKED_OFFSET + _Q_LOCKED_BITS) +#if CONFIG_NR_CPUS < (1U << 14) +#define _Q_PENDING_BITS 8 +#else #define _Q_PENDING_BITS 1 +#endif #define _Q_PENDING_MASK _Q_SET_MASK(PENDING) #define _Q_TAIL_IDX_OFFSET (_Q_PENDING_OFFSET + _Q_PENDING_BITS) @@ -61,6 +73,7 @@ typedef struct qspinlock { #define _Q_TAIL_CPU_BITS (32 - _Q_TAIL_CPU_OFFSET) #define _Q_TAIL_CPU_MASK _Q_SET_MASK(TAIL_CPU) +#define _Q_TAIL_OFFSET _Q_TAIL_IDX_OFFSET #define _Q_TAIL_MASK (_Q_TAIL_IDX_MASK | _Q_TAIL_CPU_MASK) #define _Q_LOCKED_VAL (1U << _Q_LOCKED_OFFSET) --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -22,6 +22,7 @@ #include #include #include +#include #include /* @@ -48,6 +49,9 @@ * We can further change the first spinner to spin on a bit in the lock word * instead of its node; whereby avoiding the need to carry a node from lock to * unlock, and preserving API. + * + * N.B. The current implementation only supports architectures that allow + * atomic operations on smaller 8-bit and 16-bit data types. */ #include "mcs_spinlock.h" @@ -85,6 +89,87 @@ static inline struct mcs_spinlock *decod #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK) +/* + * By using the whole 2nd least significant byte for the pending bit, we + * can allow better optimization of the lock acquisition for the pending + * bit holder. + */ +#if _Q_PENDING_BITS == 8 + +struct __qspinlock { + union { + atomic_t val; + struct { +#ifdef __LITTLE_ENDIAN + u16 locked_pending; + u16 tail; +#else + u16 tail; + u16 locked_pending; +#endif + }; + }; +}; + +/** + * clear_pending_set_locked - take ownership and clear the pending bit. + * @lock: Pointer to queue spinlock structure + * @val : Current value of the queue spinlock 32-bit word + * + * *,1,0 -> *,0,1 + * + * Lock stealing is not allowed if this function is used. + */ +static __always_inline void +clear_pending_set_locked(struct qspinlock *lock, u32 val) +{ + struct __qspinlock *l = (void *)lock; + + ACCESS_ONCE(l->locked_pending) = _Q_LOCKED_VAL; +} + +/* + * xchg_tail - Put in the new queue tail code word & retrieve previous one + * @lock : Pointer to queue spinlock structure + * @tail : The new queue tail code word + * Return: The previous queue tail code word + * + * xchg(lock, tail) + * + * p,*,* -> n,*,* ; prev = xchg(lock, node) + */ +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) +{ + struct __qspinlock *l = (void *)lock; + + return (u32)xchg(&l->tail, tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET; +} + +#else /* _Q_PENDING_BITS == 8 */ + +/** + * clear_pending_set_locked - take ownership and clear the pending bit. + * @lock: Pointer to queue spinlock structure + * @val : Current value of the queue spinlock 32-bit word + * + * *,1,0 -> *,0,1 + */ +static __always_inline void +clear_pending_set_locked(struct qspinlock *lock, u32 val) +{ + u32 new, old; + + for (;;) { + new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL; + + old = atomic_cmpxchg(&lock->val, val, new); + if (old == val) + break; + + val = old; + } +} + /** * xchg_tail - Put in the new queue tail code word & retrieve previous one * @lock : Pointer to queue spinlock structure @@ -109,6 +194,7 @@ static __always_inline u32 xchg_tail(str } return old; } +#endif /* _Q_PENDING_BITS == 8 */ /** * queue_spin_lock_slowpath - acquire the queue spinlock @@ -173,8 +259,13 @@ void queue_spin_lock_slowpath(struct qsp * we're pending, wait for the owner to go away. * * *,1,1 -> *,1,0 + * + * this wait loop must be a load-acquire such that we match the + * store-release that clears the locked bit and create lock + * sequentiality; this because not all clear_pending_set_locked() + * implementations imply full barriers. */ - while ((val = atomic_read(&lock->val)) & _Q_LOCKED_MASK) + while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_MASK) cpu_relax(); /* @@ -182,15 +273,7 @@ void queue_spin_lock_slowpath(struct qsp * * *,1,0 -> *,0,1 */ - for (;;) { - new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL; - - old = atomic_cmpxchg(&lock->val, val, new); - if (old == val) - break; - - val = old; - } + clear_pending_set_locked(lock, val); return; /*