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

[Peter Zijlstra]

On Fri, 2009-12-04 at 11:40 +0100, Peter Zijlstra wrote:
> 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.

That is, what problem do these new-fangled workqueues have and why is
this a good solution.

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