* Sebastian Andrzej Siewior <[email protected]> wrote:
> On 2017-04-06 08:16:22 [+0200], Ingo Molnar wrote:
> >
> > * Sebastian Andrzej Siewior <[email protected]> wrote:
> >
> > > On 2017-04-05 09:39:43 [+0200], Ingo Molnar wrote:
> > > >
> > > > So maybe we could add the following facility:
> > > >
> > > > ptr = sched_migrate_to_cpu_save(cpu);
> > > >
> > > > ...
> > > >
> > > > sched_migrate_to_cpu_restore(ptr);
> >
> > BTW., and I'm sure this has come up before, but why doesn't
> > migrate_disable() use
> > a simple per task flag that the scheduler migration code takes into account?
>
> we could add that. But right now there are two spots which look at the
> counter to decide whether or not migration is disabled.
>
> > It should be functionally equivalent to the current solution, and it
> > appears to
> > have a heck of a smaller cross section with the rest of the scheduler.
> >
> > I.e.:
> >
> > static inline void migrate_disable(void)
> > {
> > current->migration_disabled++;
> > }
> >
> > ...
> >
> > static inline void migrate_enable(void)
> > {
> > current->migration_disabled--;
> > }
> >
> > or so? Then add this flag as a condition to can_migrate_task() et al.
> >
> > While we generally dislike such flags as they wreck havoc with the
> > scheduler if
> > overused, the cpus_allowed based solution has the exact same effect so it's
> > not
> > like it's a step backwards - and it should also be much faster and less
> > intrusive.
>
> So you are saying that we drop the cpus_ptr + cpus_mask fields again and
> instead add a task-flag to ensure that the scheduler does not migrate
> the task to another CPU?
Yeah - but no need to add a per-task flag if we already have a counter.
> > Am I missing some complication?
>
> We do have the counter. We have need to ensure that the CPU is not going away
> while we are in a migrate_disable() region since we can be scheduled out. So
> the
> CPU can't go offline until we leave that region.
Yeah. But it should be relatively straightforward to extend the logic that
makes
sure that a CPU does not go away from under tasks pinned to that CPU alone,
right?
> #define migrate_disable() sched_migrate_to_cpu_save(-1)
>
> int sched_migrate_to_cpu_save(int cpu)
So if we have a ->migration_disabled counter then we don't need the
sched_migrate_to_cpu_save()/restore() complication, right?
Sorry if this is a back and forth - I was somehow convinced that we do need to
frob the cpus_allowed mask to get this functionality - but in hindsight I think
the counter should be enough.
I.e. just have a counter and these two APIs:
static inline void migrate_disable(void)
{
current->migration_disabled++;
}
...
static inline void migrate_enable(void)
{
current->migration_disabled--;
}
... and make sure the scheduler migration code plus the CPU hotplug code
considers
the counter.
Would this work, and would this be the simplest all around solution?
Thanks,
Ingo
