linux-kernel - Re: smp: Start up non-boot CPUs asynchronously

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Message-ID: <4F3ACC02.9020003@linux.intel.com>
Date:	Tue, 14 Feb 2012 13:02:58 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	Arjan van de Ven <arjanvandeven@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>, mikey@...ling.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	gregkh@...uxfoundation.org,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	linux-kernel@...r.kernel.org, Milton Miller <miltonm@....com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: smp: Start up non-boot CPUs asynchronously

On 2/14/2012 11:57 AM, Srivatsa S. Bhat wrote:

>> In addition to this, the reality is that the whole "bring cpus up"
>> sequence needs to be changed; the current one is very messy and requires
>> the hotplug lock for the whole bring up of each individual cpu... which
>> is a very unfortunate design; a much better design would be to only take
>> the lock for the actual registration of the newly brought up CPU to the
>> kernel, while running the physical bringup without the global lock.
>> If/when that change gets made, we can do the physical bring up in
>> parallel (with each other, but also with the rest of the kernel boot),
>> and do the registration en-mass at some convenient time in the boot,
>> potentially late.
>>
> 
> 
> Sounds like a good idea, but how will we take care of CPU_UP_PREPARE and
> CPU_STARTING callbacks then? Because, CPU_UP_PREPARE callbacks are run
> before bringing up the cpu and CPU_STARTING is called from the cpu that is
> coming up. Also, CPU_UP_PREPARE callbacks can be failed, which can lead
> to that particular cpu boot getting aborted. With the "late commissioning
> of CPUs" idea you proposed above, retaining such semantics could become
> very challenging.

some of these callbacks may need to be redesigned as well; or at least,
we may need to decouple the "physical" state of the CPU that's getting
brought up from the "logical" OS visible one.
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