Looking good. Some minor comments on these three patches ...
* The name 'nodemask' for the cpumask_t of CPUs that are siblings to CPU i
is a bit confusing (yes, that name was already there). How about
something like 'siblings' ?
* I suspect that the following two lines:
cpus_complement(cpu_default_map, cpu_isolated_map);
cpus_and(cpu_default_map, cpu_default_map, *cpu_map);
can be replaced with the one line:
cpus_andnot(cpu_default_map, *cpu_map, cpu_isolated_map);
* You have 'cpu-exclusive' in some places in the Documentation.
I would mildly prefer to always spell this 'cpu_exclusive' (with
underscore, not hyphen).
* I like how this design came out, as described in:
A cpuset that is cpu exclusive has a sched domain associated with it.
The sched domain consists of all cpus in the current cpuset that are not
part of any exclusive child cpusets.
Good work.
* Question - any idea how much of a performance hiccup a system will feel
whenever someone changes the cpu_exclusive cpusets? Could this lead
to a denial-of-service attack, if say some untrusted user were allowed
modify privileges on some small cpuset that was cpu_exclusive, and they
abused that privilege by turning on and off the cpu_exclusive property
on their little cpuset (or creating/destroying an exclusive child):
cd /dev/cpuset/$(cat /proc/self/cpuset)
while true
do
for i in 0 1
do
echo $i > cpu_exclusive
done
done
If so, perhaps we should recommend that shared systems with untrusted
users avoid allowing a cpu_exclusive cpuset to be modifiable, or to have
a cpu_exclusive flag modifiable, by those untrusted users.
* The cpuset 'oldcs' in update_flag() seems to only be used for its
cpu_exclusive flag. We could save some stack space on my favorite
big honkin NUMA iron by just having a local variable for this
'old_cpu_exclusive' value, instead of the entire cpuset.
* Similarly, though with a bit less savings, one could replace 'oldcs'
in update_cpumask() with just the old_cpus_allowed mask.
Or, skip even that, and compute a boolean flag:
cpus_changed = cpus_equal(cs->cpus_allowed, trialcs.cpus_allowed);
before copying over the trialcs, so we only need one word of stack
for the boolean, not possibly many words for a cpumask.
* Non-traditional code style:
}
else {
should be instead:
} else {
* Is it the case that update_cpu_domains() is called with cpuset_sem held?
Would it be a good idea to note in the comment for that routine:
* Call with cpuset_sem held. May nest a call to the
* lock_cpu_hotplug()/unlock_cpu_hotplug() pair.
I didn't callout the cpuset_sem lock precondition on many routines,
but since this one can nest the cpu_hotplug lock, it might be worth
calling it out, for the benefit of engineers who are passing through,
needing to know how the hotplug lock nests with other semaphores.
--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <[email protected]> 1.650.933.1373, 1.925.600.0401
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