Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

Steven Rostedt
2020-09-24T13:51:47Z

On Thu, 24 Sep 2020 14:42:41 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Sep 24, 2020 at 08:32:41AM -0400, Steven Rostedt wrote:
> > Anyway, instead of blocking. What about having a counter of number of
> > migrate disabled tasks per cpu, and when taking a migrate_disable(), and there's
> > already another task with migrate_disabled() set, and the current task has
> > an affinity greater than 1, it tries to migrate to another CPU?
>
> That doesn't solve the problem. On wakeup we should already prefer an
> idle CPU over one running a (RT) task, but you can always wake more
> tasks than there's CPUs around and you'll _have_ to stack at some point.

Yes, understood.

>
> The trick is how to unstack them correctly. We need to detect when a
> migrate_disable() task _should_ start running again, and migrate away
> whoever is in the way at that point.
>
> It turns out, that getting selected for pull-balance is exactly that
> condition, and clearly a migrate_disable() task cannot be pulled, but we
> can use that signal to try and pull away the running task that's in the
> way.

Unless of course that running task is in a migrate disable section
itself ;-)

But I guess we will always have that SHC, and there will always be a
scenario that you can't balance properly. But hopefully in practice we
wont see that.

How to handle kmap_local(), will migrate_disable() be used only for
32bit or, for consistency, will it also apply to 64bit?

-- Steve