linux-kernel - Re: [PATCH RFC tip/core/rcu 4/4] rcu: Ensure that RCU_FAST_NO

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Message-ID: <20120426155450.GD2407@linux.vnet.ibm.com>
Date:	Thu, 26 Apr 2012 08:54:50 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	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, "Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 4/4] rcu: Ensure that RCU_FAST_NO_HZ
 timers expire on correct CPU

On Thu, Apr 26, 2012 at 03:04:56PM +0200, Peter Zijlstra wrote:
> On Mon, 2012-04-23 at 09:16 -0700, Paul E. McKenney wrote:
> > -               mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
> > -                         per_cpu(rcu_idle_gp_timer_expires, cpu));
> > +               tp = &per_cpu(rcu_idle_gp_timer, cpu);
> > +               tp->expires = per_cpu(rcu_idle_gp_timer_expires, cpu);
> > +               add_timer_on(tp, cpu); 
> 
> The simpler change looks to use mod_timer_pinned()

Good point!

Except...  Now that you mention it, I don't see how mod_timer_pinned()
actually helps.  It looks to me like a CPU-hotplug operation will
migrate the timers anyway.

This is actually (in theory) harmless in the RCU_FAST_NO_HZ case, because
the CPU_DYING stuff will force a wakeup of the CPU in question, which
will cancel the timer.  But still, mod_timer_pinned() has a rather
misleading name.  ;-)

But a line is a line, so I made this change.

							Thanx, Paul

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