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Date:   Wed, 23 Sep 2020 17:12:34 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Thomas Gleixner <tglx@linutronix.de>
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of
 kmap_atomic & friends
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On Wed, 23 Sep 2020 22:55:54 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> > Perhaps make migrate_disable() an anonymous local_lock()?
> >
> > This should lower the SHC in theory, if you can't have stacked migrate
> > disables on the same CPU.  
> 
> I'm pretty sure this ends up in locking hell pretty fast and aside of
> that it's not working for scenarios like:
> 
>      kmap_local();
>        migrate_disable();
>        ...
> 
>      copy_from_user()
>         -> #PF
>            -> schedule()  
> 
> which brought us into that discussion in the first place. You would stop
> any other migrate disable user from running until the page fault is
> resolved...

Then scratch the idea of having anonymous local_lock() and just bring
local_lock in directly? Then have a kmap local lock, which would only
block those that need to do a kmap.

Now as for migration disabled nesting, at least now we would have
groupings of this, and perhaps the theorists can handle that. I mean,
how is this much different that having a bunch of tasks blocked on a
mutex with the owner is pinned on a CPU?

migrate_disable() is a BKL of pinning affinity. If we only have
local_lock() available (even on !RT), then it makes the blocking in
groups. At least this way you could grep for all the different
local_locks in the system and plug that into the algorithm for WCS,
just like one would with a bunch of mutexes.

-- Steve