From patchwork Sat Mar 2 13:14:52 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Lai Jiangshan X-Patchwork-Id: 224492 Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from ozlabs.org (localhost [IPv6:::1]) by ozlabs.org (Postfix) with ESMTP id C0EA82C032C for ; Sun, 3 Mar 2013 00:13:30 +1100 (EST) Received: from song.cn.fujitsu.com (unknown [222.73.24.84]) by ozlabs.org (Postfix) with ESMTP id A89442C02FA for ; Sun, 3 Mar 2013 00:12:56 +1100 (EST) X-IronPort-AV: E=Sophos; i="4.84,768,1355068800"; d="scan'208,223"; a="6797864" Received: from unknown (HELO tang.cn.fujitsu.com) ([10.167.250.3]) by song.cn.fujitsu.com with ESMTP; 02 Mar 2013 21:10:30 +0800 Received: from fnstmail02.fnst.cn.fujitsu.com (tang.cn.fujitsu.com [127.0.0.1]) by tang.cn.fujitsu.com (8.14.3/8.13.1) with ESMTP id r22DCoTq007867; Sat, 2 Mar 2013 21:12:51 +0800 Received: from lai.fc14.fnst ([10.167.233.241]) by fnstmail02.fnst.cn.fujitsu.com (Lotus Domino Release 8.5.3) with ESMTP id 2013030221115448-544380 ; Sat, 2 Mar 2013 21:11:54 +0800 Message-ID: <5131FB4C.7070408@cn.fujitsu.com> Date: Sat, 02 Mar 2013 21:14:52 +0800 From: Lai Jiangshan User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc14 Thunderbird/3.1.4 MIME-Version: 1.0 To: Michel Lespinasse , "Srivatsa S. Bhat" Subject: [PATCH V2] lglock: add read-preference local-global rwlock References: <512BBAD8.8010006@linux.vnet.ibm.com> <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <512E7879.20109@linux.vnet.ibm.com> <5130E8E2.50206@cn.fujitsu.com> <20130301182854.GA3631@redhat.com> In-Reply-To: X-MIMETrack: Itemize by SMTP Server on mailserver/fnst(Release 8.5.3|September 15, 2011) at 2013/03/02 21:11:54, Serialize by Router on mailserver/fnst(Release 8.5.3|September 15, 2011) at 2013/03/02 21:11:56, Serialize complete at 2013/03/02 21:11:56 Cc: Lai Jiangshan , linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, namhyung@kernel.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, Oleg Nesterov , vincent.guittot@linaro.org, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linuxppc-dev-bounces+patchwork-incoming=ozlabs.org@lists.ozlabs.org Sender: "Linuxppc-dev" From 345a7a75c314ff567be48983e0892bc69c4452e7 Mon Sep 17 00:00:00 2001 From: Lai Jiangshan Date: Sat, 2 Mar 2013 20:33:14 +0800 Subject: [PATCH] lglock: add read-preference local-global rwlock Current lglock is not read-preference, so it can't be used on some cases which read-preference rwlock can do. Example, get_cpu_online_atomic(). Although we can use rwlock for these cases which needs read-preference. but it leads to unnecessary cache-line bouncing even when there are no writers present, which can slow down the system needlessly. It will be worse when we have a lot of CPUs, it is not scale. So we look forward to lglock. lglock is read-write-lock based on percpu locks, but it is not read-preference due to its underlining percpu locks. But what if we convert the percpu locks of lglock to use percpu rwlocks: CPU 0 CPU 1 ------ ------ 1. spin_lock(&random_lock); read_lock(my_rwlock of CPU 1); 2. read_lock(my_rwlock of CPU 0); spin_lock(&random_lock); Writer: CPU 2: ------ for_each_online_cpu(cpu) write_lock(my_rwlock of 'cpu'); Consider what happens if the writer begins his operation in between steps 1 and 2 at the reader side. It becomes evident that we end up in a (previously non-existent) deadlock due to a circular locking dependency between the 3 entities, like this: (holds Waiting for random_lock) CPU 0 -------------> CPU 2 (holds my_rwlock of CPU 0 for write) ^ | | | Waiting| | Waiting for | | for | V ------ CPU 1 <------ (holds my_rwlock of CPU 1 for read) So obviously this "straight-forward" way of implementing percpu rwlocks is deadlock-prone. So we can't implement read-preference local-global rwlock like this. The implement of this patch reuse current lglock as frontend to achieve local-read-lock, and reuse global fallback rwlock as backend to achieve read-preference, and use percpu reader counter to indicate 1) the depth of the nested reader lockes and 2) whether the outmost lock is percpu lock or fallback rwlock. The algorithm is simple, in the read site: If it is nested reader, just increase the counter If it is the outmost reader, 1) try to lock its cpu's lock of the frontend lglock. (reader count +=1 if success) 2) if the above step fails, read_lock(&fallback_rwlock). (reader count += FALLBACK_BASE + 1) Write site: Do the lg_global_lock() of the frontend lglock. And then write_lock(&fallback_rwlock). Prof: 1) reader-writer exclusive: write-site must requires all percpu locks and fallback_rwlock. outmost read-site must requires one of these locks. 2) read-preference: before write site lock finished acquired, read site at least wins at read_lock(&fallback_rwlock) due to rwlock is read-preference. 3) read site functions are irqsafe(reentrance-safe) (read site functions is not protected by disabled irq, but they are irqsafe) If read site function is interrupted at any point and reenters read site again, reentranced read site will not be mislead by the first read site if the reader counter > 0, in this case, it means currently frontend(this cpu lock of lglock) or backend(fallback rwlock) lock is held, it is safe to act as nested reader. if the reader counter=0, eentranced reader considers it is the outmost read site, and it always successes after the write side release the lock. (even the interrupted read-site has already hold the cpu lock of lglock or the fallback_rwlock). And reentranced read site only calls arch_spin_trylock(), read_lock() and __this_cpu_op(), arch_spin_trylock(), read_lock() is already reentrance-safe. Although __this_cpu_op() is not reentrance-safe, but the value of the counter will be restored after the interrupted finished, so read site functions are still reentrance-safe. Performance: We only focus on the performance of the read site. this read site's fast path is just preempt_disable() + __this_cpu_read/inc() + arch_spin_trylock(), It has only one heavy memory operation. it will be expected fast. We test three locks. 1) traditional rwlock WITHOUT remote competition nor cache-bouncing.(opt-rwlock) 2) this lock(lgrwlock) 3) V6 percpu-rwlock by "Srivatsa S. Bhat". (percpu-rwlock) (https://lkml.org/lkml/2013/2/18/186) nested=1(no nested) nested=2 nested=4 opt-rwlock 517181 1009200 2010027 lgrwlock 452897 700026 1201415 percpu-rwlock 1192955 1451343 1951757 The value is the time(nano-second) of 10000 times of the operations { read-lock [nested read-lock]... [nested read-unlock]... read-unlock } If the way of this test is wrong nor bad, please correct me, the code of test is here: https://gist.github.com/laijs/5066159 (i5 760, 64bit) Changed from V1 fix a reentrance-bug which is cought by Oleg don't touch lockdep for nested reader(needed by below change) add two special APIs for cpuhotplug. Signed-off-by: Lai Jiangshan --- include/linux/lglock.h | 38 ++++++++++++++++++++++++++ kernel/lglock.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 106 insertions(+), 0 deletions(-) diff --git a/include/linux/lglock.h b/include/linux/lglock.h index 0d24e93..90bfe79 100644 --- a/include/linux/lglock.h +++ b/include/linux/lglock.h @@ -67,4 +67,42 @@ void lg_local_unlock_cpu(struct lglock *lg, int cpu); void lg_global_lock(struct lglock *lg); void lg_global_unlock(struct lglock *lg); +/* read-preference read-write-lock like rwlock but provides local-read-lock */ +struct lgrwlock { + unsigned long __percpu *reader_refcnt; + struct lglock lglock; + rwlock_t fallback_rwlock; +}; + +#define __DEFINE_LGRWLOCK_PERCPU_DATA(name) \ + static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock) \ + = __ARCH_SPIN_LOCK_UNLOCKED; \ + static DEFINE_PER_CPU(unsigned long, name ## _refcnt); + +#define __LGRWLOCK_INIT(name) \ + { \ + .reader_refcnt = &name ## _refcnt, \ + .lglock = { .lock = &name ## _lock }, \ + .fallback_rwlock = __RW_LOCK_UNLOCKED(name.fallback_rwlock)\ + } + +#define DEFINE_LGRWLOCK(name) \ + __DEFINE_LGRWLOCK_PERCPU_DATA(name) \ + struct lgrwlock name = __LGRWLOCK_INIT(name) + +#define DEFINE_STATIC_LGRWLOCK(name) \ + __DEFINE_LGRWLOCK_PERCPU_DATA(name) \ + static struct lgrwlock name = __LGRWLOCK_INIT(name) + +static inline void lg_rwlock_init(struct lgrwlock *lgrw, char *name) +{ + lg_lock_init(&lgrw->lglock, name); +} + +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw); +void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw); +void lg_rwlock_global_write_lock(struct lgrwlock *lgrw); +void lg_rwlock_global_write_unlock(struct lgrwlock *lgrw); +void __lg_rwlock_global_read_write_lock(struct lgrwlock *lgrw); +void __lg_rwlock_global_read_write_unlock(struct lgrwlock *lgrw); #endif diff --git a/kernel/lglock.c b/kernel/lglock.c index 6535a66..52e9b2c 100644 --- a/kernel/lglock.c +++ b/kernel/lglock.c @@ -87,3 +87,71 @@ void lg_global_unlock(struct lglock *lg) preempt_enable(); } EXPORT_SYMBOL(lg_global_unlock); + +#define FALLBACK_BASE (1UL << 30) + +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw) +{ + struct lglock *lg = &lgrw->lglock; + + preempt_disable(); + if (likely(!__this_cpu_read(*lgrw->reader_refcnt))) { + rwlock_acquire_read(&lg->lock_dep_map, 0, 0, _RET_IP_); + if (unlikely(!arch_spin_trylock(this_cpu_ptr(lg->lock)))) { + read_lock(&lgrw->fallback_rwlock); + __this_cpu_write(*lgrw->reader_refcnt, FALLBACK_BASE); + return; + } + } + + __this_cpu_inc(*lgrw->reader_refcnt); +} +EXPORT_SYMBOL(lg_rwlock_local_read_lock); + +void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw) +{ + switch (__this_cpu_read(*lgrw->reader_refcnt)) { + case 1: + __this_cpu_write(*lgrw->reader_refcnt, 0); + lg_local_unlock(&lgrw->lglock); + return; + case FALLBACK_BASE: + __this_cpu_write(*lgrw->reader_refcnt, 0); + read_unlock(&lgrw->fallback_rwlock); + rwlock_release(&lg->lock_dep_map, 1, _RET_IP_); + break; + default: + __this_cpu_dec(*lgrw->reader_refcnt); + break; + } + + preempt_enable(); +} +EXPORT_SYMBOL(lg_rwlock_local_read_unlock); + +void lg_rwlock_global_write_lock(struct lgrwlock *lgrw) +{ + lg_global_lock(&lgrw->lglock); + write_lock(&lgrw->fallback_rwlock); +} +EXPORT_SYMBOL(lg_rwlock_global_write_lock); + +void lg_rwlock_global_write_unlock(struct lgrwlock *lgrw) +{ + write_unlock(&lgrw->fallback_rwlock); + lg_global_unlock(&lgrw->lglock); +} +EXPORT_SYMBOL(lg_rwlock_global_write_unlock); + +/* special write-site APIs allolw nested reader in such write-site */ +void __lg_rwlock_global_read_write_lock(struct lgrwlock *lgrw) +{ + lg_rwlock_global_write_lock(lgrw); + __this_cpu_write(*lgrw->reader_refcnt, 1); +} + +void __lg_rwlock_global_read_write_unlock(struct lgrwlock *lgrw) +{ + __this_cpu_write(*lgrw->reader_refcnt, 0); + lg_rwlock_global_write_unlock(lgrw); +}