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From: peterz@infradead.org
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends
Date: Wed, 23 Sep 2020 10:40:32 +0200
Message-ID: <20200923084032.GU1362448@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <87sgbbaq0y.fsf@nanos.tec.linutronix.de>

On Mon, Sep 21, 2020 at 09:27:57PM +0200, Thomas Gleixner wrote:
> On Mon, Sep 21 2020 at 09:24, Linus Torvalds wrote:
> > On Mon, Sep 21, 2020 at 12:39 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >>
> >> If a task is migrated to a different CPU then the mapping address will
> >> change which will explode in colourful ways.
> >
> > Right you are.
> >
> > Maybe we really *could* call this new kmap functionality something
> > like "kmap_percpu()" (or maybe "local" is good enough), and make it
> > act like your RT code does for spinlocks - not disable preemption, but
> > only disabling CPU migration.
> 
> I"m all for it, but the scheduler people have opinions :)

Right, so I'm concerned. migrate_disable() wrecks pretty much all
Real-Time scheduler theory we have, and PREEMPRT_RT bringing it in is
somewhat ironic.

Yes, it allows breaking up non-preemptible regions of non-deterministic
duration, and thereby both reduce and bound the scheduling latency, the
cost for doing that is that the theory on CPU utilization/bandwidth go
out the window.

To easily see this consider an SMP system with a number of tasks equal
to the number of CPUs. On a regular (preempt_disable) kernel we can
always run each task, by virtue of always having an idle CPU to take the
task.

However, with migrate_disable() we can have each task preempted in a
migrate_disable() region, worse we can stack them all on the _same_ CPU
(super ridiculous odds, sure). And then we end up only able to run one
task, with the rest of the CPUs picking their nose.

The end result is that, like with unbounded latency, we will still miss
our deadline, simply because we got starved for CPU.


Now, while we could (with a _lot_ of work) rework the kernel to not rely
on the implicit per-cpu ness of things like spinlock_t, the moment we
bring in basic primitives that rely on migrate_disable() we're stuck
with it.


The problem is; afaict there's been no research into this problem. There
might be scheduling (read: balancing) schemes that can mitigate/solve
this problem, or it might prove to be a 'hard' problem, I just don't
know. But once we start down this road, it's going to be hell to get rid
of it.

That's why I've been arguing (for many years) to strictly limit this to
PREEMPT_RT and only as a gap-stop, not a fundamental primitive to build
on.

  reply index

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-19  9:17 Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 01/15] mm/highmem: Un-EXPORT __kmap_atomic_idx() Thomas Gleixner
2020-09-21  6:23   ` Christoph Hellwig
2020-09-19  9:17 ` [patch RFC 02/15] highmem: Provide generic variant of kmap_atomic* Thomas Gleixner
2020-09-21  6:28   ` Christoph Hellwig
2020-09-19  9:17 ` [patch RFC 03/15] x86/mm/highmem: Use generic kmap atomic implementation Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 04/15] arc/mm/highmem: " Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 05/15] ARM: highmem: Switch to generic kmap atomic Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 06/15] csky/mm/highmem: " Thomas Gleixner
2020-09-23  0:05   ` Guo Ren
2020-09-19  9:17 ` [patch RFC 07/15] microblaze/mm/highmem: " Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 08/15] mips/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 09/15] nds32/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 10/15] powerpc/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 11/15] sparc/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 12/15] xtensa/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 13/15] mm/highmem: Remove the old kmap_atomic cruft Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 14/15] sched: highmem: Store temporary kmaps in task struct Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 15/15] mm/highmem: Provide kmap_temporary* Thomas Gleixner
2020-09-19 10:35 ` [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends Daniel Vetter
2020-09-19 10:37   ` Daniel Vetter
2020-09-20  6:23     ` Thomas Gleixner
2020-09-20  8:23       ` Daniel Vetter
2020-09-20 17:24         ` Thomas Gleixner
2020-09-19 17:18 ` Linus Torvalds
2020-09-19 17:39   ` Matthew Wilcox
2020-09-19 19:13     ` Linus Torvalds
2020-09-21 19:58     ` Ira Weiny
2020-09-20  6:41   ` Thomas Gleixner
2020-09-20  8:49     ` Thomas Gleixner
2020-09-20 16:57       ` Linus Torvalds
2020-09-20 17:40         ` Thomas Gleixner
2020-09-20 17:42           ` Linus Torvalds
2020-09-20 17:58             ` Linus Torvalds
2020-09-21  7:39             ` Thomas Gleixner
2020-09-21 16:24               ` Linus Torvalds
2020-09-21 19:27                 ` Thomas Gleixner
2020-09-23  8:40                   ` peterz [this message]
2020-09-23 13:35                     ` Thomas Gleixner
2020-09-23 15:52                     ` Steven Rostedt
2020-09-23 20:55                       ` Thomas Gleixner
2020-09-23 21:12                         ` Steven Rostedt
2020-09-24  6:57                           ` Thomas Gleixner
2020-09-24 12:32                             ` Steven Rostedt
2020-09-24 12:42                               ` Peter Zijlstra
2020-09-24 13:51                                 ` Steven Rostedt
2020-09-24 13:58                                   ` Peter Zijlstra
2020-09-24 17:55                               ` Thomas Gleixner
2020-09-24 18:58                                 ` Steven Rostedt
2020-09-24  8:27                       ` peterz
2020-09-24 19:36                         ` Daniel Bristot de Oliveira
2020-09-23 10:19                   ` peterz
2020-09-23 12:33                     ` Thomas Gleixner
2020-09-23 14:33                   ` Thomas Gleixner

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