Re: [patch V4 4/8] sched: Make migrate_disable/enable() independent of RT

[Peter Zijlstra]

On Thu, Nov 19, 2020 at 09:38:34AM +0000, Mel Gorman wrote:
> On Wed, Nov 18, 2020 at 08:48:42PM +0100, Thomas Gleixner wrote:
> > From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > 
> > Now that the scheduler can deal with migrate disable properly, there is no
> > real compelling reason to make it only available for RT.
> > 
> > There are quite some code pathes which needlessly disable preemption in
> > order to prevent migration and some constructs like kmap_atomic() enforce
> > it implicitly.
> > 
> > Making it available independent of RT allows to provide a preemptible
> > variant of kmap_atomic() and makes the code more consistent in general.
> > 
> > FIXME: Rework the comment in preempt.h - Peter?
> > 
> 
> I didn't keep up to date and there is clearly a dependency on patches in
> tip for migrate_enable/migrate_disable . It's not 100% clear to me what
> reworking you're asking for but then again, I'm not Peter!

He's talking about the big one: "Migrate-Disable and why it is
undesired.".

I still hate all of this, and I really fear that with migrate_disable()
available, people will be lazy and usage will increase :/

Case at hand is this series, the only reason we need it here is because
per-cpu page-tables are expensive...

I really do think we want to limit the usage and get rid of the implicit
migrate_disable() in spinlock_t/rwlock_t for example.


AFAICT the scenario described there is entirely possible; and it has to
show up for workloads that rely on multi-cpu bandwidth for correctness.

Switching from preempt_disable() to migrate_disable() hides the
immediate / easily visible high priority latency, but you move the
interference term into a place where it is much harder to detect, you
don't lose the term, it stays in the system.


So no, I don't want to make the comment less scary. Usage is
discouraged.