Linux-csky Archive on lore.kernel.org
 help / color / Atom feed
From: Thomas Gleixner <tglx@linutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>, peterz@infradead.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Paul McKenney <paulmck@kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Will Deacon <will@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	Russell King <linux@armlinux.org.uk>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	linux-xtensa@linux-xtensa.org,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@intel.com>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	intel-gfx <intel-gfx@lists.freedesktop.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Vineet Gupta <vgupta@synopsys.com>,
	"open list\:SYNOPSYS ARC ARCHITECTURE" 
	<linux-snps-arc@lists.infradead.org>,
	Arnd Bergmann <arnd@arndb.de>, Guo Ren <guoren@kernel.org>,
	linux-csky@vger.kernel.org, Michal Simek <monstr@monstr.eu>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	linux-mips@vger.kernel.org, Nick Hu <nickhu@andestech.com>,
	Greentime Hu <green.hu@gmail.com>,
	Vincent Chen <deanbo422@gmail.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	"David S. Miller" <davem@davemloft.net>,
	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 22:55:54 +0200
Message-ID: <874kno9pr9.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200923115251.7cc63a7e@oasis.local.home>

On Wed, Sep 23 2020 at 11:52, Steven Rostedt wrote:
> 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.

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...

Thanks,

        tglx

  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
2020-09-23 13:35                     ` Thomas Gleixner
2020-09-23 15:52                     ` Steven Rostedt
2020-09-23 20:55                       ` Thomas Gleixner [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=874kno9pr9.fsf@nanos.tec.linutronix.de \
    --to=tglx@linutronix.de \
    --cc=airlied@linux.ie \
    --cc=akpm@linux-foundation.org \
    --cc=ardb@kernel.org \
    --cc=arnd@arndb.de \
    --cc=benh@kernel.crashing.org \
    --cc=bigeasy@linutronix.de \
    --cc=bristot@redhat.com \
    --cc=bsegall@google.com \
    --cc=chris@zankel.net \
    --cc=daniel@ffwll.ch \
    --cc=davem@davemloft.net \
    --cc=deanbo422@gmail.com \
    --cc=dietmar.eggemann@arm.com \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=green.hu@gmail.com \
    --cc=guoren@kernel.org \
    --cc=herbert@gondor.apana.org.au \
    --cc=intel-gfx@lists.freedesktop.org \
    --cc=jani.nikula@linux.intel.com \
    --cc=jcmvbkbc@gmail.com \
    --cc=joonas.lahtinen@linux.intel.com \
    --cc=juri.lelli@redhat.com \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-csky@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mips@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linux-snps-arc@lists.infradead.org \
    --cc=linux-xtensa@linux-xtensa.org \
    --cc=linux@armlinux.org.uk \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=mgorman@suse.de \
    --cc=monstr@monstr.eu \
    --cc=mpe@ellerman.id.au \
    --cc=nickhu@andestech.com \
    --cc=paulmck@kernel.org \
    --cc=paulus@samba.org \
    --cc=peterz@infradead.org \
    --cc=rodrigo.vivi@intel.com \
    --cc=rostedt@goodmis.org \
    --cc=sparclinux@vger.kernel.org \
    --cc=torvalds@linux-foundation.org \
    --cc=tsbogend@alpha.franken.de \
    --cc=vgupta@synopsys.com \
    --cc=vincent.guittot@linaro.org \
    --cc=will@kernel.org \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-csky Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-csky/0 linux-csky/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-csky linux-csky/ https://lore.kernel.org/linux-csky \
		linux-csky@vger.kernel.org
	public-inbox-index linux-csky

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-csky


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git