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SubjectRe: Inquiry: Should we remove "isolcpus= kernel boot option? (may have realtime uses)
Mark Hounschell wrote:
> What is an abonination, is that cpusets are equired for this type of
> isolation to begin with, even on a 2 processor machine.
> I would like the option to stay and be extended like Max originally
> proposed. If cpusets/hotplug are configured isolation would be obtained
> using them. If not then isolcpus could be used to get the same isolation.
> From a user land point of view, I just want an easy way to fully isolate
> a particular cpu. Even a new syscall or extension to sched_setaffinity
> would make me happy. Cpusets and hotplug don't.
> Again this is just MHO.

Mark, I used to be the same way and I'm a convert now. It does seems like an
overkill for 2cpu machine to have cpusets and cpu hotplug. But both options
cost around 50KB worth of text and maybe another 10KB of data. That's on the
x86-64 box. Let's say it's a 100KB. Not a terribly huge overhead.

Now if you think about it. In order to be able to dynamically isolate a cpu we
have to do exact same thing that CPU hotplug does. Which is to clear all
timers, kernel, threads, etc from that CPUs. It does not make sense to
implement a separate logic for that. You could argue that you do not need
dynamic isolation but it's too inflexible in general even on 2way machines
it's waste to not be able to use second cpu for general load even when RT app
is not running. Given that CPU hotplug is necessary for many things, including
suspend on multi-cpu machines it's practically guaranteed to be very stable
and well supported. In other words we have a perfect synergy here :).

Now, about the cpusets. You do not really have to do anything fancy with them.
If all you want to do is to disable systemwide load balancing
mount -tcgroup -o cpuset cpuset /dev/cpuset
echo 0 > /dev/cpuset/cpuset.sched_load_banace

That's it. You get _exactly_ the same effect as with isolcpus=. And you can
change that dynamically, and when you switch to quad- and eight- core machines
then you'll be to do that with groups of cpus, not just system wide.

Just to complete the example above. Lets say you want to isolate cpu2
(assuming that cpusets are already mounted).

# Bring cpu2 offline
echo 0 > /sys/devices/system/cpu/cpu2/online

# Disable system wide load balancing
echo 0 > /dev/cpuset/cpuset.sched_load_banace

# Bring cpu2 online
echo 1 > /sys/devices/system/cpu/cpu2/online

Now if you want to un-isolate cpu2 you do

# Disable system wide load balancing
echo 1 > /dev/cpuset/cpuset.sched_load_banace

Of course this is not a complete isolation. There are also irqs (see my
"default irq affinity" patch), workqueues and the stop machine. I'm working on
those too and will release .25 base cpuisol tree when I'm done.


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