Actually I have another use case here. Above example in particular may be ok
but it does demonstrate the issue nicely. Which is that in some cases kthreads
are bound to a CPU but do not have a strict "must run here" requirement and
could be moved if needed.
For example I need an ability to move workqueue threads. Workqueue threads do
kthread_bind().
So how about we add something like kthread_bind_strict() which would set
PF_THREAD_BOUND ?
We could also simply add flags argument to the kthread_bind() which would be
better imo but requires more changes. ie It'd look like
kthread_bind(..., cpu, KTHREAD_BIND_STRICT);
Things like migration threads, stop machine, etc would use the strict version
and everything else would use non-strict bind.
---
On the related note (this seems like the right crowd :). What do people think
about kthreads and cpusets in general. We currently have a bit of a disconnect
in the logic.
1. kthreads can be put into a cpuset at which point their cpus_allowed mask is
updated properly
2. kthread's cpus_allowed mask is updated properly when cpuset setup changes
(cpus added, removed, etc).
3. kthreads inherit cpuset from a parent (kthreadd for example) _but_ they
either do kthread_bind() or set_cpus_allowed() and both of those simply ignore
inherited cpusets.
Notice how scenario #3 does not fit into the overall picture. The behaviour is
inconsistent.
How about this:
- Split sched_setaffinity into
sched_setaffinity()
{
task *p = get_task_by_pid();
return task_setaffinity(p);
}
task_setaffinity(task, cpumask, flags)
{
if (flags & FORCE) {
// Used for kthreads that require strict binding.
// Detach the task from the current cpuset
// and put it into the root cpuset.
// Set PF_THREAD_BOUND.
}
// Rest of the original sched_setaffinity logic
}
- Have kthreads call task_setaffinity() instead of set_cpus_allowed() directly.
That way the behaviour will be consistent across the board.
Comments ?
Max
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