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Date:   Sat, 27 Jan 2018 15:41:54 -0800
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Lihao Liang <lihao.liang@gmail.com>
Cc:     "Guohanjun (Hanjun Guo)" <guohanjun@huawei.com>, heng.z@huawei.com,
        hb.chen@huawei.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 00/16] A new RCU implementation based on a fast
 consensus protocol
Reply-To: paulmck@linux.vnet.ibm.com
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 <20180127075744.GK3741@linux.vnet.ibm.com>
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On Fri, Jan 26, 2018 at 11:57:44PM -0800, Paul E. McKenney wrote:
> On Sat, Jan 27, 2018 at 07:22:27AM +0000, Lihao Liang wrote:
> > On Thu, Jan 25, 2018 at 5:53 AM, Paul E. McKenney
> > <paulmck@linux.vnet.ibm.com> wrote:
> > > On Tue, Jan 23, 2018 at 03:59:25PM +0800, lianglihao@huawei.com wrote:
> > >> From: Lihao Liang <lianglihao@huawei.com>
> > >>
> > >> Dear Paul,
> > >>
> > >> This patch set implements a preemptive version of RCU (PRCU) based on the following paper:
> > >>
> > >> Fast Consensus Using Bounded Staleness for Scalable Read-mostly Synchronization.
> > >> Haibo Chen, Heng Zhang, Ran Liu, Binyu Zang, and Haibing Guan.
> > >> IEEE Transactions on Parallel and Distributed Systems (TPDS), 2016.
> > >> https://dl.acm.org/citation.cfm?id=3024114.3024143
> > >>
> > >> We have also added preliminary callback-handling support.  Thus, the current version
> > >> provides APIs prcu_read_lock(), prcu_read_unlock(), synchronize_prcu(), call_prcu(),
> > >> and prcu_barrier().
> > >>
> > >> This is an experimental patch, so it would be good to have some feedback.
> > >>
> > >> Known shortcoming is that the grace-period version is incremented in synchronize_prcu().
> > >> If call_prcu() or prcu_barrier() is called but there is no synchronized_prcu() invoked,
> > >> callbacks cannot be invoked.  Later version should address this issue, e.g. adding a
> > >> grace-period expedition mechanism.  Others include to use a a hierarchical structure,
> > >> taking into account the NUMA topology, to send IPI in synchronize_prcu().
> > >>
> > >> We have tested the implementation using rcutorture on both an x86 and ARM64 machine.
> > >> PRCU passed 1h and 3h tests on all the newly added config files except PRCU07 reported BUG
> > >> in a 1h run.
> > >>
> > >> [ 1593.604201] ---[ end trace b3bae911bec86152 ]---
> > >> [ 1594.629450] prcu-torture:torture_onoff task: offlining 14
> > >> [ 1594.755553] smpboot: CPU 14 is now offline
> > >> [ 1594.757732] prcu-torture:torture_onoff task: offlined 14
> > >> [ 1597.765149] prcu-torture:torture_onoff task: onlining 11
> > >> [ 1597.766795] smpboot: Booting Node 0 Processor 11 APIC 0xb
> > >> [ 1597.804102] prcu-torture:torture_onoff task: onlined 11
> > >> [ 1599.365098] prcu-torture: rtc: ffffffffb0277b90 ver: 66358 tfle: 0 rta: 66358 rtaf: 0
> > >> rtf: 66349 rtmbe: 0 rtbe: 1 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 2233418
> > >> onoff: 191/191:199/199 34,199:59,5102 10403:0 (HZ=1000) barrier: 188/189:1 cbflood: 225
> > >> [ 1599.367946] prcu-torture: !!!
> > >> [ 1599.367966] ------------[ cut here ]------------
> > >
> > > The "rtbe: 1" indicates that your implementation of prcu_barrier()
> > > failed to wait for all preceding call_prcu() callbacks to be invoked.

And my guess is that the "rtbe: 1" is happening because you aren't doing
anything with CPU hotplug.  The way this could happen is as follows:

1.	rcutorture does call_prcu() with a test callback on CPU 5.

2.	CPU 5 goes offline, but its callbacks stay there.

3.	rcutorture does prcu_barrier(), which queues a callback on each
	sufficiently online CPU.  No callback is enqueued on CPU 5.

4.	The enqueued callbacks are all invoked, so prcu_barrier()
	returns.

5.	rcutorture complains because the test callback stranded on CPU 5
	never did get invoked.

A slightly different sequence of events could strand one of prcu_barrier()'s
callbacks on an offline CPU, in which case prcu_barrier() would hang until
that CPU came back online.  There are probably other failure scenarios, but
those two should do for now.  ;-)

							Thanx, Paul

> > > Does the immediately following "Reader Pipe:" list have any but the
> > > first two numbers non-zero?
> > 
> > Yes.
> 
> If the third or subsequent numbers are non-zero, that would indicate
> too-short grace periods.  This would be a critical bug in PRCU.
> 
> > >> We have also compared PRCU with TREE RCU using rcuperf with gp_exp set to true, that is
> > >> synchronize_rcu_expedited was tested.
> > >>
> > >> The rcuperf results are as follows (average grace-period duration in ms of ten 10min runs):
> > >>
> > >> 16*Intel Xeon CPU@2.4GHz, 16GB memory, Ubuntu Linux 3.13.0-47-generic
> > >>
> > >> CPUs      2       4       8      12      15       16
> > >> PRCU   0.14    1.07    4.15    8.02   10.79    15.16
> > >> TREE  49.30  104.75  277.55  390.82  620.82  1381.54
> > >>
> > >> 64*Cortex-A72 CPU@2.4GHz, 130GB memory, Ubuntu Linux 4.10.0-21.23-generic
> > >>
> > >> CPUs       2       4        8      16      32       48       63        64
> > >> PRCU    0.23   19.69    38.28   63.21   95.41   167.18   252.01   1841.44
> > >> TREE  416.73  901.89  1060.86  743.00  920.66  1325.21  1646.20  23806.27
> > >
> > > Well, at the very least, this is a bug report on either expedited RCU
> > > grace-period latency or on rcuperf's measurements, and thank you for that.
> > > I will look into this.  In the meantime, could you please let me know
> > > exactly how you invoked rcuperf?
> > 
> > We used the following command to invoke rcuperf:
> > 
> > sudo ./kvm.sh --torture rcuperf --duration 10 --configs 10*TREE
> > 
> > The actual script run-rcuperf.sh to run the experiments can be found
> > in the following email of this patch series:
> > 
> > [PATCH RFC 15/16] rcutorture: Add scripts to run experiments
> > 
> > Please let us know how it goes.
> 
> Will do!
> 
> As I said before, at the very least you have identified a performance bug
> in RCU expedited grace periods.
> 
> 							Thanx, Paul
> 
> > Many thanks,
> > Lihao.
> > 
> > > I have a few comments on some of your patches based on a quick scan
> > > through them.
> > >
> > >                                                         Thanx, Paul
> > >
> > >> Best wishes,
> > >> Lihao.
> > >>
> > >>
> > >> Lihao Liang (15):
> > >>   rcutorture: Add PRCU rcu_torture_ops
> > >>   rcutorture: Add PRCU test config files
> > >>   rcuperf: Add PRCU rcu_perf_ops
> > >>   rcuperf: Add PRCU test config files
> > >>   rcuperf: Set gp_exp to true for tests to run
> > >>   prcu: Implement call_prcu() API
> > >>   prcu: Implement PRCU callback processing
> > >>   prcu: Implement prcu_barrier() API
> > >>   rcutorture: Test call_prcu() and prcu_barrier()
> > >>   rcutorture: Add basic ARM64 support to run scripts
> > >>   prcu: Add PRCU Kconfig parameter
> > >>   prcu: Comment source code
> > >>   rcuperf: Add config files with various CONFIG_NR_CPUS
> > >>   rcutorture: Add scripts to run experiments
> > >>   Add GPLv2 license
> > >>
> > >> Heng Zhang (1):
> > >>   prcu: Add PRCU implementation
> > >>
> > >>  include/linux/interrupt.h                          |   3 +
> > >>  include/linux/prcu.h                               | 122 +++++
> > >>  include/linux/rcupdate.h                           |   1 +
> > >>  init/Kconfig                                       |   7 +
> > >>  init/main.c                                        |   2 +
> > >>  kernel/rcu/Makefile                                |   1 +
> > >>  kernel/rcu/prcu.c                                  | 497 +++++++++++++++++++++
> > >>  kernel/rcu/rcuperf.c                               |  33 +-
> > >>  kernel/rcu/rcutorture.c                            |  40 +-
> > >>  kernel/rcu/tree.c                                  |   1 +
> > >>  kernel/sched/core.c                                |   2 +
> > >>  kernel/time/timer.c                                |   2 +
> > >>  kvm.sh                                             | 452 +++++++++++++++++++
> > >>  run-rcuperf.sh                                     |  26 ++
> > >>  .../testing/selftests/rcutorture/bin/functions.sh  |  17 +-
> > >>  .../selftests/rcutorture/configs/rcu/CFLIST        |   5 +
> > >>  .../selftests/rcutorture/configs/rcu/PRCU02        |  27 ++
> > >>  .../selftests/rcutorture/configs/rcu/PRCU02.boot   |   1 +
> > >>  .../selftests/rcutorture/configs/rcu/PRCU03        |  23 +
> > >>  .../selftests/rcutorture/configs/rcu/PRCU03.boot   |   2 +
> > >>  .../selftests/rcutorture/configs/rcu/PRCU06        |  26 ++
> > >>  .../selftests/rcutorture/configs/rcu/PRCU06.boot   |   5 +
> > >>  .../selftests/rcutorture/configs/rcu/PRCU07        |  25 ++
> > >>  .../selftests/rcutorture/configs/rcu/PRCU07.boot   |   2 +
> > >>  .../selftests/rcutorture/configs/rcu/PRCU09        |  19 +
> > >>  .../selftests/rcutorture/configs/rcu/PRCU09.boot   |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/CFLIST    |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU      |  20 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-12   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-12.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-14   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-14.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-15   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-15.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-16   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-16.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-2    |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-2.boot         |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-32   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-32.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-4    |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-4.boot         |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-48   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-48.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-56   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-56.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-60   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-60.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-62   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-62.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-64   |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-64.boot        |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU-8    |  21 +
> > >>  .../rcutorture/configs/rcuperf/PRCU-8.boot         |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/PRCU.boot |   1 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-12   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-14   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-15   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-16   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-2    |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-32   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-4    |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-48   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-56   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-60   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-62   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-64   |  21 +
> > >>  .../selftests/rcutorture/configs/rcuperf/TREE-8    |  21 +
> > >>  68 files changed, 1918 insertions(+), 5 deletions(-)
> > >>  create mode 100644 include/linux/prcu.h
> > >>  create mode 100644 kernel/rcu/prcu.c
> > >>  create mode 100755 kvm.sh
> > >>  create mode 100755 run-rcuperf.sh
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU02
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU02.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU03
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU03.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU06
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU06.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU07
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU07.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU09
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/PRCU09.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-12
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-12.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-14
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-14.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-15
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-15.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-16
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-16.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-2
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-2.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-32
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-32.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-4
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-4.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-48
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-48.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-56
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-56.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-60
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-60.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-62
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-62.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-64
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-64.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-8
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU-8.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/PRCU.boot
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-12
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-14
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-15
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-16
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-2
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-32
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-4
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-48
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-56
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-60
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-62
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-64
> > >>  create mode 100644 tools/testing/selftests/rcutorture/configs/rcuperf/TREE-8
> > >>
> > >> --
> > >> 2.14.1.729.g59c0ea183
> > >>
> > >
> >