linux-kernel - Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU mask

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Date:   Thu, 6 Apr 2017 11:25:42 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU
 mask

On 2017-04-06 10:01:39 [+0200], Ingo Molnar wrote:
> 
> * Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > On 2017-04-06 08:16:22 [+0200], Ingo Molnar wrote:
> > > 
> > > * Sebastian Andrzej Siewior <bigeasy@...utronix.de> 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?

I used get_online_cpus() that is enough.

> > #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?

correct. Unless (as you suggested) we want a migrate to specific CPU we
just the function until the BUG() statement and don't need to check the
CPU we are on.

> 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++;
plus
	if (current->migration_disabled == 1)
		get_online_cpus()
>         }
> 
>         ...
> 
>         static inline void migrate_enable(void)
>         {
>                 current->migration_disabled--;
>         }
> 
> ... and make sure the scheduler migration code plus the CPU hotplug code considers 
> the counter.

So on the sched part I need go through all places where it looks at the
mask and make sure it ignores the CPU switch decision. This could be
doable.
On the CPU hotplug I don't see a way around get_online_cpus(). It is
essentially there to ensure a CPU does not go away. The other way around
it would be to remove the CPU from the online_mask followed by going
through all task left on the CPU waiting for them leave the CPU. And I
remember peterz saying that he wanted to make get_online_cpus() fast and
stay with that.

> Would this work, and would this be the simplest all around solution?

Let me try this with get_online_cpus() (as suggested) and without
cpus_ptr.

> Thanks,
> 
> 	Ingo

Sebastian