From: Mel Gorman <mgorman@suse.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: 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, Peter Zijlstra <peterz@infradead.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 09:38:34 +0000 [thread overview]
Message-ID: <20201119093834.GH3306@suse.de> (raw)
In-Reply-To: <20201118204007.269943012@linutronix.de>
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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!
From tip;
/**
* migrate_disable - Prevent migration of the current task
*
* Maps to preempt_disable() which also disables preemption. Use
* migrate_disable() to annotate that the intent is to prevent migration,
* but not necessarily preemption.
*
* Can be invoked nested like preempt_disable() and needs the corresponding
* number of migrate_enable() invocations.
*/
I assume that the rework is to document the distinction between
migrate_disable and preempt_disable() because it may not be clear to some
people why one should be used over another and the risk of cut&paste
cargo cult programming.
So I assume the rework is for the middle paragraph
* Maps to preempt_disable() which also disables preemption. Use
* migrate_disable() to annotate that the intent is to prevent migration,
* but not necessarily preemption. The distinction is that preemption
* disabling will protect a per-cpu structure from concurrent
* modifications due to preemption. migrate_disable partially protects
* the tasks address space and potentially preserves the TLB entries
* even if preempted such as an needed for a local IO mapping or a
* kmap_atomic() referenced by on-stack pointers to avoid interference
* between user threads or kernel threads sharing the same address space.
I know it can have other examples that are rt-specific and some tricks on
percpu page alloc draining that relies on a combination of migrate_disable
and interrupt disabling to protect the structures but the above example
might be understandable to a non-RT audience.
--
Mel Gorman
SUSE Labs
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next prev parent reply other threads:[~2020-11-19 9:38 UTC|newest]
Thread overview: 40+ 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
2020-11-18 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
2020-11-18 19:48 ` [patch V4 3/8] x86: Support kmap_local() forced debugging 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 [this message]
2020-11-19 11:14 ` Peter Zijlstra
2020-11-19 12:14 ` Mel Gorman
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
2020-11-18 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
2020-11-18 19:48 ` [patch V4 6/8] mm/highmem: Provide kmap_local* Thomas Gleixner
2020-11-18 19:48 ` [patch V4 7/8] io-mapping: Provide iomap_local variant Thomas Gleixner
2020-11-18 19:48 ` [patch V4 8/8] x86/crashdump/32: Simplify copy_oldmem_page() 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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