Message-ID: <400CE8DC.70307@cyberone.com.au>
Date: Tue, 20 Jan 2004 19:37:48 +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> <400CE354.8060300@cyberone.com.au> <20040120082943.GA15733@hockin.org>
In-Reply-To: <20040120082943.GA15733@hockin.org>
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Tim Hockin wrote:

>On Tue, Jan 20, 2004 at 07:14:12PM +1100, Nick Piggin wrote:
>
>>>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.
>>
>
>If root has a process limit that make hotplug scripts fail to run, then
>we're hosed in a lot of ways.  And if we fail to allocate memory, there
>really ought to be some retry or something.  It seems to me that a failure
>to run a hotplug script is a BAD THING.
>

(or OOM killed being another that comes to mind)

It is sometimes inevitable. With that knowledge we should be designing
for graceful failure.

>
>>>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.
>>
>
>But it is a violation of the affinity.  As the kernel we CAN NOT know what
>the affinity really means.
>

Not if the application is designed to handle it. How would hotplug
scripts make this any different, anyway?

>  Maybe there is some way for a task to indicate
>it would like to receive SIGPWR in that case.  Or some other signal.  Can we
>invent new signals?
>
>That way a task that KNOWS about the CPU disappearing underneath it can be
>wise, while everything else will not just get killed.
>

Rusty thought you just wouldn't send it unless the process was handling
it.


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