Re: [PATCH 04/40] sched: implement __set_cpus_allowed()

Peter Zijlstra
2010-01-18T09:58:02Z

On Mon, 2010-01-18 at 09:57 +0900, Tejun Heo wrote:
> set_cpus_allowed_ptr() modifies the allowed cpu mask of a task. The
> function performs the following checks before applying new mask.
>
> * Check whether PF_THREAD_BOUND is set. This is set for bound
> kthreads so that they can't be moved around.
>
> * Check whether the target cpu is still marked active - cpu_active().
> Active state is cleared early while downing a cpu.
>
> This patch adds __set_cpus_allowed() which takes @force parameter
> which when true makes __set_cpus_allowed() ignore PF_THREAD_BOUND and
> use cpu online state instead of active state for the latter. This
> allows migrating tasks to CPUs as long as they are online.
> set_cpus_allowed_ptr() is implemented as inline wrapper around
> __set_cpus_allowed().
>
> Due to the way migration is implemented, the @force parameter needs to
> be passed over to the migration thread. @force parameter is added to
> struct migration_req and passed to __migrate_task().
>
> Please note the naming discrepancy between set_cpus_allowed_ptr() and
> the new functions. The _ptr suffix is from the days when cpumask API
> wasn't mature and future changes should drop it from
> set_cpus_allowed_ptr() too.
>
> NOTE: It would be nice to implement kthread_bind() in terms of
> __set_cpus_allowed() if we can drop the capability to bind to a
> dead CPU from kthread_bind(), which doesn't seem too popular
> anyway. With such change, we'll have set_cpus_allowed_ptr() for
> regular tasks and kthread_bind() for kthreads and can use
> PF_THREAD_BOUND instead of passing @force parameter around.

And your changelog still sucks... it only says what it does, not why.

still hate the patch too.