Date: Sat, 23 Apr 2005 13:22:27 -0500
From: Nathan Lynch <ntl@pobox.com>
To: "Dr. David Alan Gilbert" <linux@treblig.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Hotplug CPU and setaffinity?
Message-ID: <20050423182227.GE18688@otto>
References: <20050423173514.GA7111@gallifrey>
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Dr. David Alan Gilbert wrote:
>
>   I got to wondering how Hotplug CPU and sched_setaffinity interact;
> if I have a process that has its affinity set to one CPU and some
> nasty person comes along and unplugs it what happens to that process-
> does it get scheduled onto another cpu, just not get any time or
> die ?

The affinity of the process is reset to the default and it is migrated
to another cpu, for better or worse.  The kernel assumes the admin
know what he/she is doing.

> 
> In particular I was thinking of the cases where a thread has a
>  functional reason for remaining on one particular CPU (e.g. if you
> had calibrated for some feature of that CPU say its time stamp
> counter skew/speed). Another case would be a set of threads which
> had set their affinity to the same CPU and then made memory
> consistency or locking assumptions that wouldn't be valid
> if they got rescheduled onto different CPUs.

Yeah.  But I don't think this is an issue to be solved in the kernel.
Applications that are this sensitive to cpu hotplugging need to
arrange to be notified before the hotplug occurs, which I think would
be best done with dbus or some other IPC.


Nathan


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