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

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Date:	Thu, 26 Apr 2012 12:08:59 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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, 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, 2012-04-26 at 08:54 -0700, Paul E. McKenney wrote:

> > 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.
> 

It's expected that if you use this (or anything else pinned to a CPU)
that you add the hotplug hooks to handle a CPU going down.

There's only two users of this that I see. One is
arch/x86/kernel/apic/x2apic_uv_x.c, that has the hotplug handling. The
other is drivers/net/ethernet/tile/tilepro.c, that does not have hotplug
handling, but the tile arch does not support hotplug anyway:

arch/tile/kernel/process.c: cpu_idle()

			if (cpu_is_offline(cpu))
				BUG();  /* no HOTPLUG_CPU */

-- Steve


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