linux-kernel - Re: [RFC PATCH 6/6] CPU hotplug: Invoke CPU offline notifiers in reverse order

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Message-ID: <20120725164339.GA32378@google.com>
Date:	Wed, 25 Jul 2012 09:43:39 -0700
From:	Tejun Heo <tj@...nel.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, mingo@...nel.org, peterz@...radead.org,
	rusty@...tcorp.com.au, paulmck@...ux.vnet.ibm.com,
	namhyung@...nel.org, rjw@...k.pl, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 6/6] CPU hotplug: Invoke CPU offline notifiers in
 reverse order

Hello,

On Wed, Jul 25, 2012 at 05:25:13PM +0530, Srivatsa S. Bhat wrote:
> During CPU hotplug, we want to create the following effect:
>  * During CPU online, the CPU incrementally grows the number of services
>    it offers.
>  * During CPU offline, the services are incrementally retired, in the
>    reverse order of their growth during CPU online.
> 
> To achieve the above, invoke the registered CPU hotplug notifiers in the
> reverse order during CPU offline, with respect to the order in which they
> were invoked during CPU online.
> 
> This approach helps in making the hotplug path a lot more logically
> coherent: Services are started in a well-known order, perhaps with dependencies
> between them, while bringing up a CPU. While offlining the CPU, we honor
> many of the dependencies automatically by just backtracking the way we came;
> that is, by invoking the notifiers in the reverse order. Thus, this model of
> reverse notifier invocation helps us in avoiding the need for excessively
> messing with the notifier priorities while trying to get the ordering right.
> 
> Notes:
> 1. The workqueue code becomes simpler, since it now needs just a single CPU
> hotplug callback.
> 2. The scheduler's sched_cpu_[in]active() callbacks can also be merged into
> a single callback.
> 3. On similar lines, the cpusets' cpuset_cpu_[in]active() callbacks can also
> be merged.
> 
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

For workqueue part,

 Acked-by: Tejun Heo <tj@...nel.org>

Now that we can reverse-walk notifier chain, it probably is a good
idea to make it a bit smarter and call CANCELs in reverse order of
PREPAREs too.

Thanks.

-- 
tejun
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