linux-kernel - Re: [RFC][PATCH 14/18] sched: Remove rq->lock from the first half of ttwu()

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Date:	Thu, 06 Jan 2011 17:29:13 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 14/18] sched: Remove rq->lock from the first half
 of ttwu()

On Tue, 2011-01-04 at 15:59 +0100, Peter Zijlstra wrote:
> Index: linux-2.6/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_fair.c
> +++ linux-2.6/kernel/sched_fair.c
> @@ -1343,8 +1343,7 @@ static void task_waking_fair(struct task
>         struct sched_entity *se = &p->se;
>         struct cfs_rq *cfs_rq = cfs_rq_of(se);
>  
> -       lockdep_assert_held(&task_rq(p)->lock);
> -
> +       // XXX racy on 32bit
>         se->vruntime -= cfs_rq->min_vruntime;
>  } 


Best I can come up with is something like this:


Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -315,6 +315,9 @@ struct cfs_rq {
 
 	u64 exec_clock;
 	u64 min_vruntime;
+#ifndef CONFIG_64BIT
+	u64 min_vruntime_copy;
+#endif
 
 	struct rb_root tasks_timeline;
 	struct rb_node *rb_leftmost;
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -365,6 +365,10 @@ static void update_min_vruntime(struct c
 	}
 
 	cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
+#ifndef CONFIG_64BIT
+	smp_wmb();
+	cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
+#endif
 }
 
 /*
@@ -1342,9 +1346,21 @@ static void task_waking_fair(struct task
 {
 	struct sched_entity *se = &p->se;
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
+	u64 min_vruntime;
 
-	// XXX racy on 32bit
-	se->vruntime -= cfs_rq->min_vruntime;
+#ifndef CONFIG_64BIT
+	u64 min_vruntime_copy;
+
+	do {
+		min_vruntime_copy = cfs_rq->min_vruntime_copy;
+		smp_rmb();
+		min_vruntime = cfs_rq->min_vruntime;
+	} while (min_vruntime != min_vruntime_copy);
+#else
+	min_vruntime = cfs_rq->min_vruntime;
+#endif
+
+	se->vruntime -= min_vruntime;
 }
 
 #ifdef CONFIG_FAIR_GROUP_SCHED

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