linux-kernel - Re: [RFC] Patch: dynticks: idle load balancing

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Message-ID: <20061213231316.GA13849@elte.hu>
Date:	Thu, 14 Dec 2006 00:13:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Cc:	nickpiggin@...oo.com.au, vatsa@...ibm.com, clameter@....com,
	tglx@...utronix.de, arjan@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] Patch: dynticks: idle load balancing


* Ingo Molnar <mingo@...e.hu> wrote:

> > Appended patch attempts to fix the process idle load balancing in 
> > the presence of dynticks. cpus for which ticks are stopped will 
> > sleep till the next event wakes it up. Potentially these sleeps can 
> > be for large durations and during which today, there is no idle load 
> > balancing being done. There was some discussion happened(last year) 
> > on this topic on lkml, where two main approaches were gettting 
> > debated. One is to back off the idle load balancing for bigger 
> > intervals and the second is a watchdog mechanism where the busy cpu 
> > will trigger the load balance on an idle cpu.  Both of these 
> > mechanisms have its drawbacks.
> 
> nice work! I have added your patch to -rt. Btw., it needs the patch 
> below to work on 64-bit.

there's another bug as well: in schedule() resched_cpu() is called with 
the current runqueue held in two places, which is deadlock potential. 
The easiest fix for this is to use trylock - find the patch for that. 
This is a hint only anyway - and if a CPU is idle its runqueue will be 
lockable. (fixing it via double-locking is easy in the first call site, 
but the second one looks harder)

	Ingo

Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -1167,12 +1167,14 @@ static void resched_task(struct task_str
 	if (!tsk_is_polling(p))
 		smp_send_reschedule(cpu);
 }
+
 static void resched_cpu(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
-	unsigned int flags;
+	unsigned long flags;
 
-	spin_lock_irqsave(&rq->lock, flags);
+	if (!spin_trylock_irqsave(&rq->lock, flags))
+		return;
 	resched_task(cpu_curr(cpu));
 	spin_unlock_irqrestore(&rq->lock, flags);
 }
-
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