From: Ankur Arora <ankur.a.arora@oracle.com>
To: paulmck@kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Ankur Arora <ankur.a.arora@oracle.com>,
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: Wed, 18 Oct 2023 13:15:28 -0700 [thread overview]
Message-ID: <87r0lroffj.fsf@oracle.com> (raw)
In-Reply-To: <61bb51f7-99ed-45bf-8c3e-f1d65137c894@paulmck-laptop>
Paul E. McKenney <paulmck@kernel.org> writes:
> On Wed, Oct 18, 2023 at 03:16:12PM +0200, Thomas Gleixner wrote:
>> Paul!
>>
>> On Tue, Oct 17 2023 at 18:03, Paul E. McKenney wrote:
>> > Belatedly calling out some RCU issues. Nothing fatal, just a
>> > (surprisingly) few adjustments that will need to be made. The key thing
>> > to note is that from RCU's viewpoint, with this change, all kernels
>> > are preemptible, though rcu_read_lock() readers remain
>> > non-preemptible.
>>
>> Why? Either I'm confused or you or both of us :)
>
> Isn't rcu_read_lock() defined as preempt_disable() and rcu_read_unlock()
> as preempt_enable() in this approach? I certainly hope so, as RCU
> priority boosting would be a most unwelcome addition to many datacenter
> workloads.
No, in this approach, PREEMPT_AUTO selects PREEMPTION and thus
PREEMPT_RCU so rcu_read_lock/unlock() would touch the
rcu_read_lock_nesting. Which is identical to what PREEMPT_DYNAMIC does.
>> With this approach the kernel is by definition fully preemptible, which
>> means means rcu_read_lock() is preemptible too. That's pretty much the
>> same situation as with PREEMPT_DYNAMIC.
>
> Please, just no!!!
>
> Please note that the current use of PREEMPT_DYNAMIC with preempt=none
> avoids preempting RCU read-side critical sections. This means that the
> distro use of PREEMPT_DYNAMIC has most definitely *not* tested preemption
> of RCU readers in environments expecting no preemption.
Ah. So, though PREEMPT_DYNAMIC with preempt=none runs with PREEMPT_RCU,
preempt=none stubs out the actual preemption via __preempt_schedule.
Okay, I see what you are saying.
(Side issue: but this means that even for PREEMPT_DYNAMIC preempt=none,
_cond_resched() doesn't call rcu_all_qs().)
>> For throughput sake this fully preemptible kernel provides a mechanism
>> to delay preemption for SCHED_OTHER tasks, i.e. instead of setting
>> NEED_RESCHED the scheduler sets NEED_RESCHED_LAZY.
>>
>> That means the preemption points in preempt_enable() and return from
>> interrupt to kernel will not see NEED_RESCHED and the tasks can run to
>> completion either to the point where they call schedule() or when they
>> return to user space. That's pretty much what PREEMPT_NONE does today.
>>
>> The difference to NONE/VOLUNTARY is that the explicit cond_resched()
>> points are not longer required because the scheduler can preempt the
>> long running task by setting NEED_RESCHED instead.
>>
>> That preemption might be suboptimal in some cases compared to
>> cond_resched(), but from my initial experimentation that's not really an
>> issue.
>
> I am not (repeat NOT) arguing for keeping cond_resched(). I am instead
> arguing that the less-preemptible variants of the kernel should continue
> to avoid preempting RCU read-side critical sections.
[ snip ]
>> 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.
As far as non-preemptible RCU read-side critical sections are concerned,
are the current
- PREEMPT_DYNAMIC=y, PREEMPT_RCU, preempt=none config
(rcu_read_lock/unlock() do not manipulate preempt_count, but do
stub out preempt_schedule())
- and PREEMPT_NONE=y, TREE_RCU config (rcu_read_lock/unlock() manipulate
preempt_count)?
roughly similar or no?
>> > I am sure that I am missing something, but I have not yet seen any
>> > show-stoppers. Just some needed adjustments.
>>
>> Right. If it works out as I think it can work out the main adjustments
>> are to remove a large amount of #ifdef maze and related gunk :)
>
> Just please don't remove the #ifdef gunk that is still needed!
Always the hard part :).
Thanks
--
ankur
next prev parent reply other threads:[~2023-10-18 20:16 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 [this message]
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
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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