linux-kernel - Re: [rcu] [ INFO: suspicious RCU usage. ]

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Message-ID: <20150204154613.GE5370@linux.vnet.ibm.com>
Date:	Wed, 4 Feb 2015 07:46:13 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Fengguang Wu <fengguang.wu@...el.com>, LKP <lkp@...org>,
	linux-kernel@...r.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	linux-arm-kernel@...ts.infradead.org,
	Arnd Bergmann <arnd@...db.de>,
	MarkRutland <mark.rutland@....com>
Subject: Re: [rcu] [ INFO: suspicious RCU usage. ]

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

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