linux-kernel - Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call

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Date:	Tue, 06 Mar 2012 16:31:10 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <eag0628@...il.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org
Subject: Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine
 call_srcu()

On Tue, 2012-03-06 at 22:44 +0800, Lai Jiangshan wrote:
> >> +                       if ((++idle_loop & 0xF) == 0) {
> >> +                               spin_unlock_irq(&sp->gp_lock);
> >> +                               udelay(1);
> >> +                               spin_lock_irq(&sp->gp_lock);
> >> +                       }
> >
> > The purpose of this bit isn't quite clear to me, is this simply a lock
> > break?
> 
> Yes, the main purpose is:
> make the time of sp->gp_lock short, can be determined. 

either introduce cond_resched_lock_irq() or write something like:

	if (need_resched() || spin_needbreak(&sp->gp_lock)) {
		spin_unlock_irq(&sp->gp_lock);
		cond_resched();
		spin_lock_irq(&sp->gp_lock);
	}

udelay(1) is complete nonsense.. 
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