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

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Date:   Thu, 6 Apr 2017 10:01:39 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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


* 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?

> #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