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Message-ID: <400CE354.8060300@cyberone.com.au>
Date: Tue, 20 Jan 2004 19:14:12 +1100
From: Nick Piggin <piggin@cyberone.com.au>
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To: Tim Hockin <thockin@hockin.org>
CC: Rusty Russell <rusty@au1.ibm.com>, vatsa@in.ibm.com,
       linux-kernel@vger.kernel.org, torvalds@osdl.org, akpm@osdl.org,
       rml@tech9.net
Subject: Re: CPU Hotplug: Hotplug Script And SIGPWR
References: <20040116174446.A2820@in.ibm.com> <20040120060027.91CC717DE5@ozlabs.au.ibm.com> <20040120063316.GA9736@hockin.org> <400CCE2F.2060502@cyberone.com.au> <20040120065207.GA10993@hockin.org> <400CD4B5.6020507@cyberone.com.au> <20040120073032.GB12638@hockin.org> <400CDCA1.5070200@cyberone.com.au> <20040120075409.GA13897@hockin.org>
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Tim Hockin wrote:

>On Tue, Jan 20, 2004 at 06:45:37PM +1100, Nick Piggin wrote:
>
>>>I guess a hotplug script MAY fail.  I don't think it's a good idea to make
>>>your CPU hotplug script fail.  May and Misght are different.  It's up to 
>>>the
>>>implementor whether the script can get into a failure condition.
>>>
>>>
>>Sorry bad wording. The script may fail to be executed.
>>
>
>Under what conditions?  Not arbitrary entropy, surely.  If a hotplug script
>is present and does not blow up, it should be safe to assume it will be run
>upon an event being delivered.  If not, we have a WAY bigger problem :)
>

That assumption is not safe. The main problems are of course process limits
and memory allocation failure.

>
>>>What if <which> process needs guaranteed scheduling latency?  Do we really
>>>_guarantee_ scheduling latency *anywhere*?
>>>
>>We do guarantee that a realtime task won't be blocked waiting for
>>a hotplug script to fault in and start it up again (which may not
>>happen). Not sure how important this issue is.
>>
>
>We have a conflict of priority here.  If an RT task is affined to CPU A and
>CPU A gets yanked out, what do we do?
>
>Obviously the RT task can't keep running as it was.  It was affined to A.
>Maybe for a good reason.  I see we have a few choices here:
>
>* re-affine it automatically, thereby silently undoing the explicit
>  affinity.
>* violate it's RT scheduling by not running it until it has been re-affined
>  or CPU A returns to the pool/
>
>Sending it a SIGPWR means you have to run it on a different CPU that it was
>affined to, which is already a violation.
>

At least the task has the option to handle the problem.


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