Re: CPU Hotplug: Hotplug Script And SIGPWR

[Nick Piggin <piggin@cyberone.com.au>]

Tim Hockin wrote:

>On Tue, Jan 20, 2004 at 04:44:45PM +1100, Rusty Russell wrote:
>
>>The other issue I wanted to revisit: we currently send SIGPWR to all
>>processes which we have to undo the CPU affinity for (with a new
>>si_info field containing the cpu going down).
>>
>>The main problem is that a process can call sched_setaffinity on
>>another (unrelated) task, which might not know about it.  One option
>>would be to only deliver the signal if it's not SIG_DFL for that
>>process.  Another would be not to signal, and expect hotplug scripts
>>to clean up.
>>
>
>I had to deal with this in my procstate patch (was against RH 2.4 with O(1)
>sched but not 2.6).  What I chose to do (and what the people who were
>wanting the code wanted) was to move tasks which had no CPU to run upon onto
>an unrunnable list.  Whenever a CPU's state is changed, scan the list.
>Whenevr a task's affinity mask is changed, check if it needs to go onto or
>come off of the unrunnable_list.
>
>I added a new TASK_UNRUNNABLE state for these tasks, too.  By adding the
>task's current (or most recent) CPU and the task's cpus_allowed and
>cpus_allowed_mask to /proc/pid/status, we gave simple tools for finding
>these unrunnable tasks.
>
>I think the sanest thing for a CPU removal is to migrate everything off the
>processor in question, move unrunnable tasks into TASK_UNRUNNABLE state,
>then notify /sbin/hotplug.  The hotplug script can then find and handle the
>unrunnable tasks.  No SIGPWR grossness needed.
>
>Code against 2.4 at http://www.hockin.org/~thockin/procstate - it was
>heavily tested and I *think* it is all correct (for that kernel snapshot).
>


Seems less robust and more ad hoc than SIGPWR, however.