Re: [PATCH RT 7/8] sched: migrate_enable: Use select_fallback_rq()

[Scott Wood]

On Tue, 2019-09-17 at 18:00 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-07-27 00:56:37 [-0500], Scott Wood wrote:
> > migrate_enable() currently open-codes a variant of select_fallback_rq().
> > However, it does not have the "No more Mr. Nice Guy" fallback and thus
> > it will pass an invalid CPU to the migration thread if cpus_mask only
> > contains a CPU that is !active.
> > 
> > Signed-off-by: Scott Wood <swood@xxxxxxxxxx>
> > ---
> > This scenario will be more likely after the next patch, since
> > the migrate_disable_update check goes away.  However, it could happen
> > anyway if cpus_mask was updated to a CPU other than the one we were
> > pinned to, and that CPU subsequently became inactive.
> 
> I'm unclear about the problem / side effect this has (before and after
> the change). It is possible (before and after that change) that a CPU is
> selected which is invalid / goes offline after the "preempt_enable()"
> statement and before stop_one_cpu() does its job, correct?

By "pass an invalid CPU" I don't mean offline; I mean >= nr_cpu_ids which is
what cpumask_any_and() returns if the sets don't intersect (a CPU going
offline is merely a way to end up in that situation).  At one point I
observed that causing a crash.  I guess is_cpu_allowed() returned true by
chance based on the contents of data beyond the end of the cpumask?  That
doesn't seem likely based on what comes after p->cpus_mask (at that point
migrate_disable should be zero), but maybe I had something else at that
point in the struct while developing.  In any case, not something to be
relied on. :-)

Going offline after selection shouldn't be a problem.  migration_cpu_stop()
won't do anything if is_cpu_allowed() returns false, and we'll get migrated
off the CPU by migrate_tasks().  Even if we get migrated after reading
task_cpu(p) but before queuing the stop machine work, it'll either get
flushed when the stopper thread parks, or rejected due to stopper->enabled
being false.

-Scott