From: "Paul E. McKenney" <paulmck@kernel.org>
To: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de,
dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com,
juri.lelli@redhat.com, vincent.guittot@linaro.org,
willy@infradead.org, mgorman@suse.de, rostedt@goodmis.org,
jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com,
boris.ostrovsky@oracle.com, konrad.wilk@oracle.com,
jgross@suse.com, andrew.cooper3@citrix.com,
Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
Date: Fri, 20 Oct 2023 19:08:03 -0700 [thread overview]
Message-ID: <d4817916-a32f-4846-a1d9-86a4f54cee53@paulmck-laptop> (raw)
In-Reply-To: <87r0loiy3y.fsf@oracle.com>
On Fri, Oct 20, 2023 at 06:05:21PM -0700, Ankur Arora wrote:
>
> Paul E. McKenney <paulmck@kernel.org> writes:
>
> > On Fri, Oct 20, 2023 at 03:56:38PM -0700, Ankur Arora wrote:
> >>
> >> Paul E. McKenney <paulmck@kernel.org> writes:
> >>
> >> > Thomas!
> >> >
> >> > On Thu, Oct 19, 2023 at 02:21:35AM +0200, Thomas Gleixner wrote:
> >> >> Paul!
> >> >>
> >> >> On Wed, Oct 18 2023 at 10:19, Paul E. McKenney wrote:
> >> >> > On Wed, Oct 18, 2023 at 03:16:12PM +0200, Thomas Gleixner wrote:
> >> >> >> On Tue, Oct 17 2023 at 18:03, Paul E. McKenney wrote:
> >> >> >> In the end there is no CONFIG_PREEMPT_XXX anymore. The only knob
> >> >> >> remaining would be CONFIG_PREEMPT_RT, which should be renamed to
> >> >> >> CONFIG_RT or such as it does not really change the preemption
> >> >> >> model itself. RT just reduces the preemption disabled sections with the
> >> >> >> lock conversions, forced interrupt threading and some more.
> >> >> >
> >> >> > Again, please, no.
> >> >> >
> >> >> > There are situations where we still need rcu_read_lock() and
> >> >> > rcu_read_unlock() to be preempt_disable() and preempt_enable(),
> >> >> > repectively. Those can be cases selected only by Kconfig option, not
> >> >> > available in kernels compiled with CONFIG_PREEMPT_DYNAMIC=y.
> >> >>
> >> >> Why are you so fixated on making everything hardcoded instead of making
> >> >> it a proper policy decision problem. See above.
> >> >
> >> > Because I am one of the people who will bear the consequences.
> >> >
> >> > In that same vein, why are you so opposed to continuing to provide
> >> > the ability to build a kernel with CONFIG_PREEMPT_RCU=n? This code
> >> > is already in place, is extremely well tested, and you need to handle
> >> > preempt_disable()/preeempt_enable() regions of code in any case. What is
> >> > the real problem here?
> >>
>
> [ snip ]
>
> >> As far as I can tell (which isn't all that far), TREE_RCU=y makes strictly
> >> stronger forward progress guarantees with respect to rcu readers (in
> >> that they can't be preempted.)
> >
> > TREE_RCU=y is absolutely required if you want a kernel to run on a system
> > with more than one CPU, and for that matter, if you want preemptible RCU,
> > even on a single-CPU system.
> >
> >> So, can PREEMPTION=y run with, say TREE_RCU=y? Or maybe I'm missing something
> >> obvious there.
> >
> > If you meant to ask about PREEMPTION and PREEMPT_RCU, in theory, you
> > can run any combination:
>
> Sorry, yes I did. Should have said "can PREEMPTION=y run with, (TREE_RCU=y,
> PREEMPT_RCU=n).
>
> > PREEMPTION && PREEMPT_RCU: This is what we use today for preemptible
> > kernels, so this works just fine (famous last words).
> >
> > PREEMPTION && !PREEMPT_RCU: A preemptible kernel with non-preemptible
> > RCU, so that rcu_read_lock() is preempt_disable() and
> > rcu_read_unlock() is preempt_enable(). This should just work,
> > except for the fact that cond_resched() disappears, which
> > stymies some of RCU's forward-progress mechanisms. And this
> > was the topic of our earlier discussion on this thread. The
> > fixes should not be too hard.
> >
> > Of course, this has not been either tested or used for at least
> > eight years, so there might be some bitrot. If so, I will of
> > course be happy to help fix it.
> >
> >
> > !PREEMPTION && PREEMPT_RCU: A non-preemptible kernel with preemptible
> > RCU. Although this particular combination of Kconfig
> > options has not been tested for at least eight years, giving
> > a kernel built with CONFIG_PREEMPT_DYNAMIC=y the preempt=none
> > kernel boot parameter gets you pretty close. Again, there is
> > likely to be some bitrot somewhere, but way fewer bits to rot
> > than for PREEMPTION && !PREEMPT_RCU. Outside of the current
> > CONFIG_PREEMPT_DYNAMIC=y case, I don't see the need for this
> > combination, but if there is a need and if it is broken, I will
> > be happy to help fix it.
> >
> > !PREEMPTION && !PREEMPT_RCU: A non-preemptible kernel with non-preemptible
> > RCU, which is what we use today for non-preemptible kernels built
> > with CONFIG_PREEMPT_DYNAMIC=n. So to repeat those famous last
> > works, this works just fine.
> >
> > Does that help, or am I missing the point of your question?
>
> It does indeed. What I was going for, is that this series (or, at
> least my adaptation of TGLX's PoC) wants to keep CONFIG_PREEMPTION
> in spirit, while doing away with it as a compile-time config option.
>
> That it does, as TGLX mentioned upthread, by moving all of the policy
> to the scheduler, which can be tuned by user-space (via sched-features.)
>
> So, my question was in response to this:
>
> >> > In that same vein, why are you so opposed to continuing to provide
> >> > the ability to build a kernel with CONFIG_PREEMPT_RCU=n? This code
> >> > is already in place, is extremely well tested, and you need to handle
> >> > preempt_disable()/preeempt_enable() regions of code in any case. What is
> >> > the real problem here?
>
> Based on your response the (PREEMPT_RCU=n, TREE_RCU=y) configuration
> seems to be eminently usable with this configuration.
>
> (Or maybe I'm missed the point of that discussion.)
>
> On a related note, I had started rcutorture on a (PREEMPTION=y, PREEMPT_RCU=n,
> TREE_RCU=y) kernel some hours ago. Nothing broken (yet!).
Thank you, and here is hoping! ;-)
Thanx, Paul
next prev parent reply other threads:[~2023-10-21 2:21 UTC|newest]
Thread overview: 152+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-30 18:49 [PATCH v2 0/9] x86/clear_huge_page: multi-page clearing Ankur Arora
2023-08-30 18:49 ` [PATCH v2 1/9] mm/clear_huge_page: allow arch override for clear_huge_page() Ankur Arora
2023-08-30 18:49 ` [PATCH v2 2/9] mm/huge_page: separate clear_huge_page() and copy_huge_page() Ankur Arora
2023-08-30 18:49 ` [PATCH v2 3/9] mm/huge_page: cleanup clear_/copy_subpage() Ankur Arora
2023-09-08 13:09 ` Matthew Wilcox
2023-09-11 17:22 ` Ankur Arora
2023-08-30 18:49 ` [PATCH v2 4/9] x86/clear_page: extend clear_page*() for multi-page clearing Ankur Arora
2023-09-08 13:11 ` Matthew Wilcox
2023-08-30 18:49 ` [PATCH v2 5/9] x86/clear_page: add clear_pages() Ankur Arora
2023-08-30 18:49 ` [PATCH v2 6/9] x86/clear_huge_page: multi-page clearing Ankur Arora
2023-08-31 18:26 ` kernel test robot
2023-09-08 12:38 ` Peter Zijlstra
2023-09-13 6:43 ` Raghavendra K T
2023-08-30 18:49 ` [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED Ankur Arora
2023-09-08 7:02 ` Peter Zijlstra
2023-09-08 17:15 ` Linus Torvalds
2023-09-08 22:50 ` Peter Zijlstra
2023-09-09 5:15 ` Linus Torvalds
2023-09-09 6:39 ` Ankur Arora
2023-09-09 9:11 ` Peter Zijlstra
2023-09-09 20:04 ` Ankur Arora
2023-09-09 5:30 ` Ankur Arora
2023-09-09 9:12 ` Peter Zijlstra
2023-09-09 20:15 ` Ankur Arora
2023-09-09 21:16 ` Linus Torvalds
2023-09-10 3:48 ` Ankur Arora
2023-09-10 4:35 ` Linus Torvalds
2023-09-10 10:01 ` Ankur Arora
2023-09-10 18:32 ` Linus Torvalds
2023-09-11 15:04 ` Peter Zijlstra
2023-09-11 16:29 ` andrew.cooper3
2023-09-11 17:04 ` Ankur Arora
2023-09-12 8:26 ` Peter Zijlstra
2023-09-12 12:24 ` Phil Auld
2023-09-12 12:33 ` Matthew Wilcox
2023-09-18 23:42 ` Thomas Gleixner
2023-09-19 1:57 ` Linus Torvalds
2023-09-19 8:03 ` Ingo Molnar
2023-09-19 8:43 ` Ingo Molnar
2023-09-19 13:43 ` Thomas Gleixner
2023-09-19 13:25 ` Thomas Gleixner
2023-09-19 12:30 ` Thomas Gleixner
2023-09-19 13:00 ` Arches that don't support PREEMPT Matthew Wilcox
2023-09-19 13:34 ` Geert Uytterhoeven
2023-09-19 13:37 ` John Paul Adrian Glaubitz
2023-09-19 13:42 ` Peter Zijlstra
2023-09-19 13:48 ` John Paul Adrian Glaubitz
2023-09-19 14:16 ` Peter Zijlstra
2023-09-19 14:24 ` John Paul Adrian Glaubitz
2023-09-19 14:32 ` Matthew Wilcox
2023-09-19 15:31 ` Steven Rostedt
2023-09-20 14:38 ` Anton Ivanov
2023-09-21 12:20 ` Arnd Bergmann
2023-09-19 14:17 ` Thomas Gleixner
2023-09-19 14:50 ` H. Peter Anvin
2023-09-19 14:57 ` Matt Turner
2023-09-19 17:09 ` Ulrich Teichert
2023-09-19 17:25 ` Linus Torvalds
2023-09-19 17:58 ` John Paul Adrian Glaubitz
2023-09-19 18:31 ` Thomas Gleixner
2023-09-19 18:38 ` Steven Rostedt
2023-09-19 18:52 ` Linus Torvalds
2023-09-19 19:53 ` Thomas Gleixner
2023-09-20 7:32 ` Ingo Molnar
2023-09-20 7:29 ` Ingo Molnar
2023-09-20 8:26 ` Thomas Gleixner
2023-09-20 10:37 ` David Laight
2023-09-19 14:21 ` Anton Ivanov
2023-09-19 15:17 ` Thomas Gleixner
2023-09-19 15:21 ` Anton Ivanov
2023-09-19 16:22 ` Richard Weinberger
2023-09-19 16:41 ` Anton Ivanov
2023-09-19 17:33 ` Thomas Gleixner
2023-10-06 14:51 ` Geert Uytterhoeven
2023-09-20 14:22 ` [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED Ankur Arora
2023-09-20 20:51 ` Thomas Gleixner
2023-09-21 0:14 ` Thomas Gleixner
2023-09-21 0:58 ` Ankur Arora
2023-09-21 2:12 ` Thomas Gleixner
2023-09-20 23:58 ` Thomas Gleixner
2023-09-21 0:57 ` Ankur Arora
2023-09-21 2:02 ` Thomas Gleixner
2023-09-21 4:16 ` Ankur Arora
2023-09-21 13:59 ` Steven Rostedt
2023-09-21 16:00 ` Linus Torvalds
2023-09-21 22:55 ` Thomas Gleixner
2023-09-23 1:11 ` Thomas Gleixner
2023-10-02 14:15 ` Steven Rostedt
2023-10-02 16:13 ` Thomas Gleixner
2023-10-18 1:03 ` Paul E. McKenney
2023-10-18 12:09 ` Ankur Arora
2023-10-18 17:51 ` Paul E. McKenney
2023-10-18 22:53 ` Thomas Gleixner
2023-10-18 23:25 ` Paul E. McKenney
2023-10-18 13:16 ` Thomas Gleixner
2023-10-18 14:31 ` Steven Rostedt
2023-10-18 17:55 ` Paul E. McKenney
2023-10-18 18:00 ` Steven Rostedt
2023-10-18 18:13 ` Paul E. McKenney
2023-10-19 12:37 ` Daniel Bristot de Oliveira
2023-10-19 17:08 ` Paul E. McKenney
2023-10-18 17:19 ` Paul E. McKenney
2023-10-18 17:41 ` Steven Rostedt
2023-10-18 17:59 ` Paul E. McKenney
2023-10-18 20:15 ` Ankur Arora
2023-10-18 20:42 ` Paul E. McKenney
2023-10-19 0:21 ` Thomas Gleixner
2023-10-19 19:13 ` Paul E. McKenney
2023-10-20 21:59 ` Paul E. McKenney
2023-10-20 22:56 ` Ankur Arora
2023-10-20 23:36 ` Paul E. McKenney
2023-10-21 1:05 ` Ankur Arora
2023-10-21 2:08 ` Paul E. McKenney [this message]
2023-10-24 12:15 ` Thomas Gleixner
2023-10-24 18:59 ` Paul E. McKenney
2023-09-23 22:50 ` Thomas Gleixner
2023-09-24 0:10 ` Thomas Gleixner
2023-09-24 7:19 ` Matthew Wilcox
2023-09-24 7:55 ` Thomas Gleixner
2023-09-24 10:29 ` Matthew Wilcox
2023-09-25 0:13 ` Ankur Arora
2023-10-06 13:01 ` Geert Uytterhoeven
2023-09-19 7:21 ` Ingo Molnar
2023-09-19 19:05 ` Ankur Arora
2023-10-24 14:34 ` Steven Rostedt
2023-10-25 1:49 ` Steven Rostedt
2023-10-26 7:50 ` Sergey Senozhatsky
2023-10-26 12:48 ` Steven Rostedt
2023-09-11 16:48 ` Steven Rostedt
2023-09-11 20:50 ` Linus Torvalds
2023-09-11 21:16 ` Linus Torvalds
2023-09-12 7:20 ` Peter Zijlstra
2023-09-12 7:38 ` Ingo Molnar
2023-09-11 22:20 ` Steven Rostedt
2023-09-11 23:10 ` Ankur Arora
2023-09-11 23:16 ` Steven Rostedt
2023-09-12 16:30 ` Linus Torvalds
2023-09-12 3:27 ` Matthew Wilcox
2023-09-12 16:20 ` Linus Torvalds
2023-09-19 3:21 ` Andy Lutomirski
2023-09-19 9:20 ` Thomas Gleixner
2023-09-19 9:49 ` Ingo Molnar
2023-08-30 18:49 ` [PATCH v2 8/9] irqentry: define irqentry_exit_allow_resched() Ankur Arora
2023-09-08 12:42 ` Peter Zijlstra
2023-09-11 17:24 ` Ankur Arora
2023-08-30 18:49 ` [PATCH v2 9/9] x86/clear_huge_page: make clear_contig_region() preemptible Ankur Arora
2023-09-08 12:45 ` Peter Zijlstra
2023-09-03 8:14 ` [PATCH v2 0/9] x86/clear_huge_page: multi-page clearing Mateusz Guzik
2023-09-05 22:14 ` Ankur Arora
2023-09-08 2:18 ` Raghavendra K T
2023-09-05 1:06 ` Raghavendra K T
2023-09-05 19:36 ` Ankur Arora
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