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From: Steven Rostedt <rostedt@goodmis.org>
To: peterz@infradead.org
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
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	linux-sparc <sparclinux@vger.kernel.org>
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends
Date: Wed, 23 Sep 2020 11:52:51 -0400	[thread overview]
Message-ID: <20200923115251.7cc63a7e@oasis.local.home> (raw)
In-Reply-To: <20200923084032.GU1362448@hirez.programming.kicks-ass.net>

On Wed, 23 Sep 2020 10:40:32 +0200
peterz@infradead.org wrote:

> 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.

What if we just made migrate_disable() a local_lock() available for !RT?

I mean make it a priority inheritance PER CPU lock.

That is, no two tasks could do a migrate_disable() on the same CPU? If
one task does a migrate_disable() and then gets preempted and the
preempting task does a migrate_disable() on the same CPU, it will block
and wait for the first task to do a migrate_enable().

No two tasks on the same CPU could enter the migrate_disable() section
simultaneously, just like no two tasks could enter a preempt_disable()
section.

In essence, we just allow local_lock() to be used for both RT and !RT.

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.

-- Steve

  parent reply	other threads:[~2020-09-23 15:53 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-19  9:17 [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends 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
2020-09-23 13:35                     ` Thomas Gleixner
2020-09-23 15:52                     ` Steven Rostedt [this message]
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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