From: Mel Gorman <mgorman@suse.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
x86@kernel.org, Linus Torvalds <torvalds@linuxfoundation.org>,
Christoph Hellwig <hch@lst.de>,
Matthew Wilcox <willy@infradead.org>,
Daniel Vetter <daniel@ffwll.ch>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Ingo Molnar <mingo@kernel.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [patch V4 4/8] sched: Make migrate_disable/enable() independent of RT
Date: Thu, 19 Nov 2020 12:14:13 +0000 [thread overview]
Message-ID: <20201119121413.GI3306@suse.de> (raw)
In-Reply-To: <20201119111411.GL3121378@hirez.programming.kicks-ass.net>
On Thu, Nov 19, 2020 at 12:14:11PM +0100, Peter Zijlstra wrote:
> 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@linutronix.de>
> > >
> > > 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.".
>
Ah yes, that makes more sense. I was thinking in terms of what is protected
but the PREEMPT_RT hazard is severe.
> 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 guessed, it was the only thing that made sense.
> 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.
More scary then by adding this to the kerneldoc section for
migrate_disable?
* Usage of migrate_disable is heavily discouraged as it is extremely
* hazardous on PREEMPT_RT kernels and any usage needs to be heavily
* justified. Before even thinking about using this, read
* "Migrate-Disable and why it is undesired" in
* include/linux/preempt.h and include both a comment and document
* in the changelog why the use case is an exception.
It's not necessary for the current series because the interface hides
it and anyone poking at the internals of kmap_atomic probably should be
aware of the address space and TLB hazards associated with it. There are
few in-tree users and presumably any future preempt-rt related merges
already know why migrate_disable is required.
However, with the kerneldoc, there is no excuse for missing it for new
users that are not PREEMPT_RT-aware. It makes it easier to NAK/revert a
patch without proper justification similar to how undocumented usages of
memory barriers tend to get a poor reception.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2020-11-19 12:15 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-18 19:48 [patch V4 0/8] mm/highmem: Preemptible variant of kmap_atomic & friends Thomas Gleixner
2020-11-18 19:48 ` [patch V4 1/8] mm/highmem: Provide and use CONFIG_DEBUG_KMAP_LOCAL Thomas Gleixner
2020-11-24 14:20 ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 2/8] mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP Thomas Gleixner
2020-11-18 21:13 ` Linus Torvalds
2020-11-19 8:46 ` Mel Gorman
2020-11-19 17:19 ` Linus Torvalds
2020-11-24 14:20 ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 3/8] x86: Support kmap_local() forced debugging Thomas Gleixner
2020-11-24 14:20 ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2021-01-06 23:01 ` [BUG] from " Steven Rostedt
2021-01-07 1:03 ` Linus Torvalds
2021-01-07 1:16 ` Steven Rostedt
2021-01-07 1:49 ` Steven Rostedt
2021-01-07 1:49 ` Jakub Kicinski
2021-01-07 2:11 ` Willem de Bruijn
2021-01-07 4:44 ` Willem de Bruijn
2021-01-07 19:47 ` Linus Torvalds
2021-01-07 20:52 ` Steven Rostedt
2021-01-07 21:07 ` Willem de Bruijn
2020-11-18 19:48 ` [patch V4 4/8] sched: Make migrate_disable/enable() independent of RT Thomas Gleixner
2020-11-19 9:38 ` Mel Gorman
2020-11-19 11:14 ` Peter Zijlstra
2020-11-19 12:14 ` Mel Gorman [this message]
2020-11-19 14:17 ` Steven Rostedt
2020-11-19 17:23 ` Linus Torvalds
2020-11-19 18:28 ` Peter Zijlstra
2020-11-20 1:33 ` Thomas Gleixner
2020-11-20 9:29 ` Peter Zijlstra
2020-11-22 23:16 ` Andy Lutomirski
2020-11-23 21:15 ` Thomas Gleixner
2020-11-23 21:25 ` Thomas Gleixner
2020-11-23 22:07 ` Andy Lutomirski
2020-11-23 23:10 ` Thomas Gleixner
2020-11-24 10:29 ` [tip: sched/core] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 5/8] sched: highmem: Store local kmaps in task struct Thomas Gleixner
2020-11-19 11:33 ` Peter Zijlstra
2020-11-19 11:51 ` Peter Zijlstra
2020-11-19 12:12 ` Peter Zijlstra
2020-11-19 14:11 ` Thomas Gleixner
2020-11-24 14:20 ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 6/8] mm/highmem: Provide kmap_local* Thomas Gleixner
2020-11-24 14:20 ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 7/8] io-mapping: Provide iomap_local variant Thomas Gleixner
2020-11-24 14:20 ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 8/8] x86/crashdump/32: Simplify copy_oldmem_page() Thomas Gleixner
2020-11-24 14:20 ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-24 8:03 ` [patch V4 0/8] mm/highmem: Preemptible variant of kmap_atomic & friends Peter Zijlstra
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