From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C6913C35247 for ; Mon, 3 Feb 2020 14:33:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A2EA22082E for ; Mon, 3 Feb 2020 14:33:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728455AbgBCOd2 (ORCPT ); Mon, 3 Feb 2020 09:33:28 -0500 Received: from relay.sw.ru ([185.231.240.75]:42134 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726268AbgBCOd1 (ORCPT ); Mon, 3 Feb 2020 09:33:27 -0500 Received: from dhcp-172-16-24-104.sw.ru ([172.16.24.104]) by relay.sw.ru with esmtp (Exim 4.92.3) (envelope-from ) id 1iycmd-0006Ed-DN; Mon, 03 Feb 2020 17:33:03 +0300 Subject: Re: [PATCH -v2 5/7] locking/percpu-rwsem: Remove the embedded rwsem To: Peter Zijlstra Cc: mingo@kernel.org, will@kernel.org, oleg@redhat.com, tglx@linutronix.de, linux-kernel@vger.kernel.org, bigeasy@linutronix.de, juri.lelli@redhat.com, williams@redhat.com, bristot@redhat.com, longman@redhat.com, dave@stgolabs.net, jack@suse.com References: <20200131150703.194229898@infradead.org> <20200131151540.155211856@infradead.org> <7a876b46-b80c-1164-d139-6026adcb222c@virtuozzo.com> <20200203134441.GI14914@hirez.programming.kicks-ass.net> From: Kirill Tkhai Message-ID: <0b035f3f-35fe-542f-8120-b57d7b66eb36@virtuozzo.com> Date: Mon, 3 Feb 2020 17:33:02 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.2 MIME-Version: 1.0 In-Reply-To: <20200203134441.GI14914@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03.02.2020 16:44, Peter Zijlstra wrote: > Hi Kirill, > > On Mon, Feb 03, 2020 at 02:45:16PM +0300, Kirill Tkhai wrote: > >> Maybe, this is not a subject of this patchset. But since this is a newborn function, >> can we introduce it to save one unneeded wake_up of writer? This is a situation, >> when writer becomes woken up just to write itself into sem->writer.task. >> >> Something like below: >> >> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c >> index a136677543b4..e4f88bfd43ed 100644 >> --- a/kernel/locking/percpu-rwsem.c >> +++ b/kernel/locking/percpu-rwsem.c >> @@ -9,6 +9,8 @@ >> #include >> #include >> >> +static bool readers_active_check(struct percpu_rw_semaphore *sem); >> + >> int __percpu_init_rwsem(struct percpu_rw_semaphore *sem, >> const char *name, struct lock_class_key *key) >> { >> @@ -101,6 +103,16 @@ static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader) >> return __percpu_down_write_trylock(sem); >> } >> >> +static void queue_sem_writer(struct percpu_rw_semaphore *sem, struct task_struct *p) >> +{ >> + rcu_assign_pointer(sem->writer.task, p); >> + smp_mb(); >> + if (readers_active_check(sem)) { >> + WRITE_ONCE(sem->writer.task, NULL); >> + wake_up_process(p); >> + } >> +} >> + >> /* >> * The return value of wait_queue_entry::func means: >> * >> @@ -129,7 +141,11 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, >> list_del_init(&wq_entry->entry); >> smp_store_release(&wq_entry->private, NULL); >> >> - wake_up_process(p); >> + if (reader || readers_active_check(sem)) >> + wake_up_process(p); >> + else >> + queue_sem_writer(sem, p); >> + >> put_task_struct(p); >> >> return !reader; /* wake (readers until) 1 writer */ >> @@ -247,8 +263,11 @@ void percpu_down_write(struct percpu_rw_semaphore *sem) >> * them. >> */ >> >> - /* Wait for all active readers to complete. */ >> - rcuwait_wait_event(&sem->writer, readers_active_check(sem)); >> + if (rcu_access_pointer(sem->writer.task)) >> + WRITE_ONCE(sem->writer.task, NULL); >> + else >> + /* Wait for all active readers to complete. */ >> + rcuwait_wait_event(&sem->writer, readers_active_check(sem)); >> } >> EXPORT_SYMBOL_GPL(percpu_down_write); >> >> Just an idea, completely untested. > > Hurm,.. I think I see what you're proposing. I also think your immediate > patch is racy, consider for example what happens if your > queue_sem_writer() finds !readers_active_check(), such that we do in > fact need to wait. Then your percpu_down_write() will find > sem->writer.task and clear it -- no waiting. You mean, down_read() wakes up waiters unconditionally. So, optimization in percpu_down_write() will miss readers_active_check() check. You are sure. Then we have to modify this a little bit and to remove the optimization from percpu_down_write(): diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c index a136677543b4..90647ab28804 100644 --- a/kernel/locking/percpu-rwsem.c +++ b/kernel/locking/percpu-rwsem.c @@ -9,6 +9,8 @@ #include #include +static bool readers_active_check(struct percpu_rw_semaphore *sem); + int __percpu_init_rwsem(struct percpu_rw_semaphore *sem, const char *name, struct lock_class_key *key) { @@ -101,6 +103,16 @@ static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader) return __percpu_down_write_trylock(sem); } +static void queue_sem_writer(struct percpu_rw_semaphore *sem, struct task_struct *p) +{ + rcu_assign_pointer(sem->writer.task, p); + smp_mb(); + if (readers_active_check(sem)) { + WRITE_ONCE(sem->writer.task, NULL); + wake_up_process(p); + } +} + /* * The return value of wait_queue_entry::func means: * @@ -129,7 +141,11 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, list_del_init(&wq_entry->entry); smp_store_release(&wq_entry->private, NULL); - wake_up_process(p); + if (reader || readers_active_check(sem)) + wake_up_process(p); + else + queue_sem_writer(sem, p); + put_task_struct(p); return !reader; /* wake (readers until) 1 writer */ @@ -248,6 +264,7 @@ void percpu_down_write(struct percpu_rw_semaphore *sem) */ /* Wait for all active readers to complete. */ + /* sem->writer is NULL or points to current */ rcuwait_wait_event(&sem->writer, readers_active_check(sem)); } EXPORT_SYMBOL_GPL(percpu_down_write); > Also, I'm not going to hold up these patches for this, we can always do > this on top. > > Still, let me consider this a little more. No problem, this is just an idea.