From mboxrd@z Thu Jan  1 00:00:00 1970
From: paulmck@linux.vnet.ibm.com (Paul E. McKenney)
Date: Wed, 4 Feb 2015 07:46:13 -0800
Subject: [rcu] [ INFO: suspicious RCU usage. ]
In-Reply-To: <20150204151624.GI8656@n2100.arm.linux.org.uk>
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 <20150204130018.GG8656@n2100.arm.linux.org.uk>
 <20150204131420.GC5370@linux.vnet.ibm.com>
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 <20150204151624.GI8656@n2100.arm.linux.org.uk>
Message-ID: <20150204154613.GE5370@linux.vnet.ibm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Wed, Feb 04, 2015 at 03:16:24PM +0000, Russell King - ARM Linux wrote:
> On Wed, Feb 04, 2015 at 07:10:28AM -0800, Paul E. McKenney wrote:
> > You know, this situation is giving me a bad case of nostalgia for the
> > old Sequent Symmetry and NUMA-Q hardware.  On those platforms, the
> > outgoing CPU could turn itself off, and thus didn't need to tell some
> > other CPU when it was ready to be turned off.  Seems to me that this
> > self-turn-off capability would be a great feature for future systems!
> 
> Unfortunately, some briliant people decided that secure firmware on
> their platforms (which is sometimes needed to turn the secondary CPUs
> off) can only be called by CPU0...
> 
> Other people decide that they can power down the secondary CPU when it
> hits a WFI (wait for interrupt) instruction after arming that state
> change, which is far saner - but we still need to know on the requesting
> CPU when the dying CPU has completed the time-expensive parts of the
> offlining process.

I suppose that you could grant the outgoing CPU the ability to arm
that state, but easy for me to say...

Anyway, still looks like a pure polling loop is required, with short
timed waits running on the surviving CPU.

							Thanx, Paul