linux-kernel - Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil

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Message-ID: <20181129213421.wwvhsjql3m3lvtv4@linux-r8p5>
Date:   Thu, 29 Nov 2018 13:34:21 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>,
        Yongji Xie <elohimes@...il.com>, mingo@...hat.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        xieyongji@...du.com, zhangyu31@...du.com, liuqi16@...du.com,
        yuanlinsi01@...du.com, nixun@...du.com, lilin24@...du.com,
        longman@...hat.com
Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the
 reader waiter to nil

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);
>