Re: Hotplug CPU and setaffinity?

["Dave Gilbert (Home)" <gilbertd@treblig.org>]

Joel Schopp wrote:

> On ppc64 we have CPU guard, which would remove a processor if it is 
> failing.  Of course, the implications of not removing such a CPU are 
> pretty terrible.

Indeed.

>>>> 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.
> 
> 
> This sounds like a theoretical problem.  Can you think of any real 
> examples?  The only cases I can think of cause performance hits, but not 
> functional problems.

Well I'm not aware of anything that currently would break with it; but I 
was gently thinking of whether it would be possible to read cycle 
counters (as a faster gettimeofday) even on systems which had 
unsynchronised counters if you could lock the thread that did it to a 
particular physical cpu.
But this behaviour currently makes that a bad idea; in this case it 
would be nicer if the kernel either just killed my process or just 
unscheduled it or sent it a signal.

Dave