From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Pingfan Liu <kernelfans@gmail.com>,
LKML <linux-kernel@vger.kernel.org>, rcu <rcu@vger.kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
Neeraj Upadhyay <quic_neeraju@quicinc.com>,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Price <steven.price@arm.com>,
Mark Rutland <mark.rutland@arm.com>,
Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@linux.intel.com>,
"Jason A. Donenfeld" <Jason@zx2c4.com>,
boqun.feng@gmail.com
Subject: Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel
Date: Wed, 24 Aug 2022 12:21:29 -0700 [thread overview]
Message-ID: <20220824192129.GE6159@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <de70b840-df04-5a52-c36f-9eaed839aa7c@joelfernandes.org>
On Wed, Aug 24, 2022 at 01:26:01PM -0400, Joel Fernandes wrote:
>
>
> On 8/24/2022 12:20 PM, Paul E. McKenney wrote:
> > On Wed, Aug 24, 2022 at 09:53:11PM +0800, Pingfan Liu wrote:
> >> On Tue, Aug 23, 2022 at 11:01 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> >>>
> >>> On Tue, Aug 23, 2022 at 09:50:56AM +0800, Pingfan Liu wrote:
> >>>> On Sun, Aug 21, 2022 at 07:45:28PM -0700, Paul E. McKenney wrote:
> >>>>> On Mon, Aug 22, 2022 at 10:15:16AM +0800, Pingfan Liu wrote:
> >>>>>> In order to support parallel, rcu_state.n_online_cpus should be
> >>>>>> atomic_dec()
> >>>>>>
> >>>>>> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> >>>>>
> >>>>> I have to ask... What testing have you subjected this patch to?
> >>>>>
> >>>>
> >>>> This patch subjects to [1]. The series aims to enable kexec-reboot in
> >>>> parallel on all cpu. As a result, the involved RCU part is expected to
> >>>> support parallel.
> >>>
> >>> I understand (and even sympathize with) the expectation. But results
> >>> sometimes diverge from expectations. There have been implicit assumptions
> >>> in RCU about only one CPU going offline at a time, and I am not sure
> >>> that all of them have been addressed. Concurrent CPU onlining has
> >>> been looked at recently here:
> >>>
> >>> https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing
> >>>
> >>> You did us atomic_dec() to make rcu_state.n_online_cpus decrementing be
> >>> atomic, which is good. Did you look through the rest of RCU's CPU-offline
> >>> code paths and related code paths?
> >>
> >> I went through those codes at a shallow level, especially at each
> >> cpuhp_step hook in the RCU system.
> >
> > And that is fine, at least as a first step.
> >
> >> But as you pointed out, there are implicit assumptions about only one
> >> CPU going offline at a time, I will chew the google doc which you
> >> share. Then I can come to a final result.
> >
> > Boqun Feng, Neeraj Upadhyay, Uladzislau Rezki, and I took a quick look,
> > and rcu_boost_kthread_setaffinity() seems to need some help. As it
> > stands, it appears that concurrent invocations of this function from the
> > CPU-offline path will cause all but the last outgoing CPU's bit to be
> > (incorrectly) set in the cpumask_var_t passed to set_cpus_allowed_ptr().
> >
> > This should not be difficult to fix, for example, by maintaining a
> > separate per-leaf-rcu_node-structure bitmask of the concurrently outgoing
> > CPUs for that rcu_node structure. (Similar in structure to the
> > ->qsmask field.)
> >
> > There are probably more where that one came from. ;-)
>
> Should rcutree_dying_cpu() access to rnp->qsmask have a READ_ONCE() ? I was
> thinking grace period initialization or qs reporting paths racing with that. Its
> just tracing, still :)
Looks like it should be regardless of Pingfan's patches, given that
the grace-period kthread might report a quiescent state concurrently.
Good catch!
Thanx, Paul
next prev parent reply other threads:[~2022-08-24 19:21 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-22 2:15 [RFC 00/10] arm64/riscv: Introduce fast kexec reboot Pingfan Liu
2022-08-22 2:15 ` [RFC 01/10] cpu/hotplug: Make __cpuhp_kick_ap() ready for async Pingfan Liu
2022-08-22 2:15 ` [RFC 02/10] cpu/hotplug: Compile smp_shutdown_nonboot_cpus() conditioned on CONFIG_SHUTDOWN_NONBOOT_CPUS Pingfan Liu
2022-08-22 2:15 ` [RFC 03/10] cpu/hotplug: Introduce fast kexec reboot Pingfan Liu
2022-08-22 2:15 ` [RFC 04/10] cpu/hotplug: Check the capability of kexec quick reboot Pingfan Liu
2022-08-22 2:15 ` [RFC 05/10] perf/arm-dsu: Make dsu_pmu_cpu_teardown() parallel Pingfan Liu
2022-08-22 2:15 ` [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel Pingfan Liu
2022-08-22 2:45 ` Paul E. McKenney
2022-08-23 1:50 ` Pingfan Liu
2022-08-23 3:01 ` Paul E. McKenney
2022-08-24 13:53 ` Pingfan Liu
2022-08-24 16:20 ` Paul E. McKenney
2022-08-24 17:26 ` Joel Fernandes
2022-08-24 19:21 ` Paul E. McKenney [this message]
2022-08-24 22:54 ` Joel Fernandes
2022-08-24 23:01 ` Paul E. McKenney
2022-08-31 16:15 ` Paul E. McKenney
2022-09-05 3:53 ` Pingfan Liu
2022-09-06 18:45 ` Paul E. McKenney
2022-08-22 18:08 ` Joel Fernandes
2022-08-23 1:56 ` Pingfan Liu
2022-08-23 3:14 ` Joel Fernandes
2022-08-24 13:38 ` Pingfan Liu
2022-08-24 13:44 ` Jason A. Donenfeld
2022-08-22 2:15 ` [RFC 07/10] lib/cpumask: Introduce cpumask_not_dying_but() Pingfan Liu
2022-08-22 14:15 ` Yury Norov
2022-08-23 7:29 ` Pingfan Liu
2022-08-22 2:15 ` [RFC 08/10] cpuhp: Replace cpumask_any_but(cpu_online_mask, cpu) Pingfan Liu
2022-08-22 2:15 ` [RFC 09/10] genirq/cpuhotplug: Ask migrate_one_irq() to migrate to a real online cpu Pingfan Liu
2022-08-22 2:15 ` [RFC 10/10] arm64: smp: Make __cpu_disable() parallel Pingfan Liu
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=20220824192129.GE6159@paulmck-ThinkPad-P17-Gen-1 \
--to=paulmck@kernel.org \
--cc=Jason@zx2c4.com \
--cc=boqun.feng@gmail.com \
--cc=frederic@kernel.org \
--cc=jiangshanlai@gmail.com \
--cc=joel@joelfernandes.org \
--cc=josh@joshtriplett.org \
--cc=kernelfans@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mathieu.desnoyers@efficios.com \
--cc=quic_neeraju@quicinc.com \
--cc=rcu@vger.kernel.org \
--cc=rostedt@goodmis.org \
--cc=sathyanarayanan.kuppuswamy@linux.intel.com \
--cc=steven.price@arm.com \
--cc=tglx@linutronix.de \
/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).