From: peterz@infradead.org
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
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: Thu, 24 Sep 2020 10:27:17 +0200 [thread overview]
Message-ID: <20200924082717.GA1362448@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200923115251.7cc63a7e@oasis.local.home>
On Wed, Sep 23, 2020 at 11:52:51AM -0400, 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?
Can't, neiter migrate_disable() nor migrate_enable() are allowed to
block -- which is what makes their implementation so 'interesting'.
> This should lower the SHC in theory, if you can't have stacked migrate
> disables on the same CPU.
See this email in that other thread (you're on Cc too IIRC):
https://lkml.kernel.org/r/20200923170809.GY1362448@hirez.programming.kicks-ass.net
I think that is we 'frob' the balance PULL, we'll end up with something
similar.
Whichever way around we turn this thing, the migrate_disable() runtime
(we'll have to add a tracer for that), will be an interference term on
the lower priority task, exactly like preempt_disable() would be. We'll
just not exclude a higher priority task from running.
AFAICT; the best case is a single migrate_disable() nesting, where a
higher priority task preempts in a migrate_disable() section -- this is
per design.
When this preempted task becomes elegible to run under the ideal model
(IOW it becomes one of the M highest priority tasks), it might still
have to wait for the preemptee's migrate_disable() section to complete.
Thereby suffering interference in the exact duration of
migrate_disable() section.
Per this argument, the change from preempt_disable() to
migrate_disable() gets us:
- higher priority tasks gain reduced wake-up latency
- lower priority tasks are unchanged and are subject to the exact same
interference term as if the higher priority task were using
preempt_disable().
Since we've already established this term is unbounded, any task but the
highest priority task is basically buggered.
TL;DR, if we get balancing fixed and achieve (near) the optimal case
above, migrate_disable() is an over-all win. But it's provably
non-deterministic as long as the migrate_disable() sections are
non-deterministic.
The reason this all mostly works in practise is (I think) because:
- People care most about the higher prio RT tasks and craft them to
mostly avoid the migrate_disable() infected code.
- The preemption scenario is most pronounced at higher utilization
scenarios, and I suspect this is fairly rare to begin with.
- And while many of these migrate_disable() regions are unbound in
theory, in practise they're often fairly reasonable.
So my current todo list is:
- Change RT PULL
- Change DL PULL
- Add migrate_disable() tracer; exactly like preempt/irqoff, except
measuring task-runtime instead of cpu-time.
- Add a mode that measures actual interference.
- Add a traceevent to detect preemption in migrate_disable().
And then I suppose I should twist Daniel's arm to update his model to
include these scenarios and numbers.
next prev parent reply other threads:[~2020-09-24 8:28 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
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 [this message]
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=20200924082717.GA1362448@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--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=rodrigo.vivi@intel.com \
--cc=rostedt@goodmis.org \
--cc=sparclinux@vger.kernel.org \
--cc=tglx@linutronix.de \
--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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).