Re: [PATCH 4/7] sched: implement force_cpus_allowed()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2009-12-02 at 12:56 +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 force_cpus_allowed() which bypasses the above two
> checks.  The caller is responsible for guaranteeing that the
> destination cpu doesn't go down until force_cpus_allowed() finishes.
> 
> The first check is bypassed by factoring out actual migration part
> into __set_cpus_allowed() from set_cpus_allowed_ptr() and calling the
> inner function from force_cpus_allowed().
> 
> The second check is buried deep down in __migrate_task() which is
> executed by migration threads.  @force parameter is added to
> __migrate_task().  As the only way to pass parameters from
> __set_cpus_allowed() is through migration_req, migration_req->force is
> added and the @force parameter is passed down 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.
> 
> force_cpus_allowed() will be used for concurrency-managed workqueue.

Would still like to know why all this is needed.