From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>,
Yongji Xie <elohimes@gmail.com>,
mingo@redhat.com, will.deacon@arm.com,
linux-kernel@vger.kernel.org, xieyongji@baidu.com,
zhangyu31@baidu.com, liuqi16@baidu.com, yuanlinsi01@baidu.com,
nixun@baidu.com, lilin24@baidu.com, longman@redhat.com
Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil
Date: Thu, 29 Nov 2018 13:34:21 -0800 [thread overview]
Message-ID: <20181129213421.wwvhsjql3m3lvtv4@linux-r8p5> (raw)
In-Reply-To: <20181129213017.v3eljor54lfpoug2@linux-r8p5>
I messed up something such that waiman was not in the thread. Ccing.
>On Thu, 29 Nov 2018, Waiman Long wrote:
>
>>That can be costly for x86 which will now have 2 locked instructions.
>
>Yeah, and when used as an actual queue we should really start to notice.
>Some users just have a single task in the wake_q because avoiding the cost
>of wake_up_process() with locks held is significant.
>
>How about instead of adding the barrier before the cmpxchg, we do it
>in the failed branch, right before we return. This is the uncommon
>path.
>
>Thanks,
>Davidlohr
>
>diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>index 091e089063be..0d844a18a9dc 100644
>--- a/kernel/sched/core.c
>+++ b/kernel/sched/core.c
>@@ -408,8 +408,14 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> * This cmpxchg() executes a full barrier, which pairs with the full
> * barrier executed by the wakeup in wake_up_q().
> */
>- if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
>+ if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL)) {
>+ /*
>+ * Ensure, that when the cmpxchg() fails, the corresponding
>+ * wake_up_q() will observe our prior state.
>+ */
>+ smp_mb__after_atomic();
> return;
>+ }
>
> get_task_struct(task);
>
next prev parent reply other threads:[~2018-11-29 21:34 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-29 12:50 [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil Yongji Xie
2018-11-29 13:12 ` Peter Zijlstra
2018-11-29 13:44 ` Peter Zijlstra
2018-11-29 14:02 ` Yongji Xie
2018-11-29 18:43 ` Davidlohr Bueso
2018-11-29 18:49 ` Waiman Long
2018-11-29 15:21 ` Waiman Long
2018-11-29 15:29 ` Waiman Long
2018-11-29 16:06 ` Peter Zijlstra
2018-11-29 17:02 ` Waiman Long
2018-11-29 17:27 ` Peter Zijlstra
2018-11-29 17:58 ` Waiman Long
2018-11-29 18:13 ` Peter Zijlstra
2018-11-29 18:17 ` Davidlohr Bueso
2018-11-29 18:08 ` Peter Zijlstra
2018-11-29 18:26 ` Waiman Long
2018-11-29 18:31 ` Will Deacon
2018-11-29 18:34 ` Waiman Long
2018-11-29 22:05 ` Peter Zijlstra
2018-11-30 9:34 ` 答复: " Liu,Qi(ACU-T1)
2018-11-30 14:15 ` Peter Zijlstra
2018-11-29 21:30 ` Davidlohr Bueso
2018-11-29 21:34 ` Davidlohr Bueso [this message]
2018-11-29 22:17 ` Peter Zijlstra
2018-11-30 9:30 ` Andrea Parri
2018-12-03 5:31 ` [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg Davidlohr Bueso
2018-12-03 16:10 ` Waiman Long
2019-01-21 11:28 ` [tip:locking/core] sched/wake_q: Add branch prediction hint to " tip-bot for Davidlohr Bueso
2018-12-10 15:12 ` [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil Yongji Xie
2018-12-17 11:37 ` Peter Zijlstra
2018-12-17 13:12 ` Yongji Xie
2019-01-07 14:35 ` Waiman Long
2019-01-07 15:31 ` Peter Zijlstra
2019-01-07 15:35 ` Waiman Long
2018-12-17 20:53 ` Davidlohr Bueso
2018-12-18 13:10 ` Peter Zijlstra
2018-12-18 13:14 ` Peter Zijlstra
2018-12-18 17:27 ` Davidlohr Bueso
2018-12-18 18:54 ` [PATCH v2] sched/wake_q: Reduce reference counting for special users Davidlohr Bueso
2018-12-18 19:17 ` Waiman Long
2018-12-18 19:30 ` Davidlohr Bueso
2018-12-18 19:39 ` Davidlohr Bueso
2018-12-18 19:53 ` [PATCH v4] " Davidlohr Bueso
2018-12-18 20:35 ` Waiman Long
2019-01-21 16:02 ` Davidlohr Bueso
2019-01-22 8:55 ` Peter Zijlstra
2019-02-04 8:57 ` [tip:locking/core] " tip-bot for Davidlohr Bueso
2019-02-07 19:30 ` Davidlohr Bueso
2019-02-12 14:14 ` Daniel Vacek
2019-01-21 11:28 ` [tip:locking/core] locking/rwsem: Fix (possible) missed wakeup tip-bot for Xie Yongji
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