Re: cpusets and kthreads, inconsistent behaviour

[David Rientjes]

On Tue, 10 Jun 2008, Max Krasnyansky wrote:

> Basically the issue is that current behaviour of the cpusets is inconsistent
> with regards to kthreads. Kthreads inherit cpuset from a parent properly but
> they simply ignore cpuset.cpus when their cpu affinity is set/updated.
> I think the behaviour must be consistent across the board. cpuset.cpus must
> apply to _all_ the tasks in the set, not just some of the tasks. If kthread
> must run on the cpus other than current_cpuset.cpus then it should detach from
> the cpuset.
> 

I disagree that a cpuset's set of allowable cpus should apply to all tasks 
attached to that cpuset.  It's certainly beneficial to be able to further 
constrict the set of allowed cpus for a task using sched_setaffinity().

It makes more sense to argue that for each task p, p->cpus_allowed is a 
subset of task_cs(p)->cpus_allowed.

> To give you an example kthreads like scsi_eh, kswapd, kacpid, pdflush,
> kseriod, etc are all started with cpus_allows=ALL_CPUS even though they
> inherit a cpuset from kthreadd. Yes they can moved manually (with
> sched_setaffinity) but the behaviour is not consistent, and for no good
> reason. kthreads can be stopped/started at any time (module load for example)
> which means that the user will have to keep moving them.
> 

This doesn't seem to be purely a kthread issue.  Tasks can be moved to a 
disjoint set of cpus by any caller to set_cpus_allowed_ptr() in the 
kernel.

		David
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